A Study of Automotive Gear Lubes
Introduction
The requirements for automotive gear lubrication have changed over the years, yet
vehicle owner awareness has not. Gear lubrication has been commonly considered elementary,
but, in fact, it is a dynamic process that requires sophisticated technology. The
differentials that house the gears are out of sight, out of mind. They are neglected.
But differentials are just as important to the operation of a vehicle as the engine.
An engine without a functioning differential will not move the vehicle. Gear lubrication
needs to be taken more seriously than before. There are several forces driving the
need for better gear lubrication.
First, is improved fuel economy. Modern vehicle aerodynamics, with lower level air
dams, is decreasing the air flow over differentials. Fuel economy is improved, but
reduced air flow increases differential operating temperatures. Also, lubricant
fill volumes in differentials have been reduced in order to lower fluid drag on
the gears and bearings for further improvement in fuel economy. However, lubricants
cool components, and with less fluid in the sump, operating temperatures rise.
Improvements in vehicle performance have created additional need for more sophisticated
gear lubrication. Model-year 2007 turbo diesel pickup trucks, V-10 gasoline pickups
and sport utility vehicles (SUVs), and high-horsepower V-8 trucks have more towing
and payload capacities than in previous years, yet their differentials have not
changed. There has been a 34% increase in engine horsepower over the last decade,
while axle gear sizes have remained constant, sump capacities have been lowered
and drain intervals extended. In the light truck segment there has been a 93% horsepower
increase since 1981.1 In vehicles such as a fifth-wheel equipped Ford F-350 Super
Duty, towing capacities have reached a high of 19,200 lbs.2 And testing shows that
in new axle applications simulating trailer towing at 88 km/h (55 mph) at a 3.5%
grade temperatures can reach as high as 188°C (370°F).3 Stress on differentials
has also increased in limousines, conversion vans, and trucks and cars with modified,
high-performance engines. More power, more towing capacity and higher hauling limits
greatly increase the stress that causes heat and wear.
Improvements in vehicle comfort have also driven the need for better gear lubrication.
The demand for greater interior space has forced vehicle manufacturers to lower
floor boards, which restricts air flow to the differential. Hot exhaust systems
are forced closer to the axle housing, and differential operating temperatures are
increased even further.
Most vehicles operate under severe service as defined by vehicle manufacturers,
but the majority of vehicle owners are unaware of this. Severe service applications
include towing, hauling, plowing, off-road use, frequent stop-and-go driving, steep-hill
driving and temperature extremes. Severe service applications are on the rise. For example, more than 90 percent of Ford Super Duty pickups are used for towing.4 Severe
service increases the need for better gear lubrication.
Synthetic gear lubes are recognized as superior to petroleum-based gear lubes by
vehicle manufacturers, gear manufacturers and most high-performance automotive experts.
Synthetic gear lubes exhibit all-around better performance. There are many synthetic
gear lubricants available to consumers, including those marketed by vehicle manufacturers.
All position themselves as superior to the rest.
Operating Conditions and Lubrication Requirements
Differentials contain many different components, each having its own requirements
for lubrication. The ring and pinion gears operate under extreme pressure and sliding
contact that require extreme-pressure additives for protection. The bearings operate
under rolling motion where lubricant film strength is particularly important, and
limited-slip clutches require special friction additives for proper operation. It
is essential, therefore, that gear lube formulations be carefully balanced to protect
all components. Too much emphasis on the needs of one component can detract from
the needs of another.
Purpose
The purpose of this paper is to inform consumers about the increasingly severe conditions
under which differentials operate and to provide data reflecting the quality and
cost differences of popular synthetic and petroleum gear lubes. With this information,
consumers are better prepared to make informed decisions when purchasing gear lubricants.
Method
The testing by which the gear lubes were evaluated was done in accordance with American
Society for Testing and Materials (ASTM) procedures, Society of Automotive Engineers
(SAE) J306 requirements and Federal Test Method Standards. Other than the oxidation
filter patch procedure, performance testing was conducted by an independent laboratory.
Physical-property testing (viscosity, viscosity index, pour point and foaming after
oxidation) was conducted in-house. A notarized affidavit certifying that the results
are accurately reported is included in Appendix 1. Gear lube pricing was obtained
from the manufacturers or distributors, and a notarized affidavit certifying that
those prices are reported as obtained is included in Appendix 2.
Scope
The focus of this paper is on American Petroleum Institute (API) GL-5, SAE 75W-90
synthetic gear lubes. Samples of API GL-5, SAE 80W-90 petroleum gear lubes were
also included for comparative purposes. The tests were selected to measure the properties
consistent with extreme-pressure gear lubricant requirements and are intended to
reveal the lubricants’ overall performance. The performance characteristics evaluated
include each gear lube’s ability to:
1. Meet the required viscosity grade of an application
2. Maintain viscosity when subjected to temperature changes
3. Retain viscosity during use
4. Function in cold temperatures
5. Resist high temperatures and oxidation
6. Protect under extreme pressures
7. Protect against wear
8. Resist foaming
9. Prevent copper corrosion.
Review Candidates
The cross-section of gear lubricants tested includes those offered by original equipment
manufacturers (OEMs), motor oil companies and specialty companies. All gear lubes,
with the exception of Mopar Synthetic and Torco SGO Synthetic, are recommended by
their manufacturers for limited-slip differentials and are therefore expected to
contain appropriate limitedslip- type additives. Mopar limited-slip additive was
added to Mopar Synthetic and Torco Type G limited-slip additive was added to Torco
SGO Synthetic at the recommended levels to ensure equal testing. Each gear lube
tested is listed in the following chart along with the performance specifications
identified on the respective bottles. Batch codes are also listed.
Gear lubricant specifications are established for minimum performance levels. The
active API gear lubricant specifications are API GL-4, GL-5 and MT-1. API GL-4 designates
the type of service characteristics of spiral bevel and hypoid gears in automotive
axles operated under moderate speeds and loads. These gear lubes may be used in
select manual transmissions and transaxles. API MT-1 designates lubricants for non-synchromesh
manual transmissions and transaxles. API MT- 1 is independent of API GL-5. API MT-1
calls for a higher level of oxidation stability, copper corrosion resistance and
seal compatibility, which is not provided by API GL-4 or GL-5. Not all gear lubes
meet API MT-1 performance standards.
API GL-1, GL-2, GL-3 and GL-6 are inactive. API GL-6 is identified by Lucas, Red
Line and Torco as a performance specification. However, the test equipment is obsolete.
The U.S. military has established separate gear lube specifications. The most current
military specification is MIL-PRF- 2105E, which supersedes the previous specification,
MIL-L-2105D. MIL-PRF-2105E combines the performance requirements of MIL-L-2105D,
API GL-5 and all but one parameter of API MT-1, thereby adding improved oxidation
stability, copper corrosion resistance and seal compatibility to extreme-pressure
axle lubricants. An additional gear lube standard, SAE J2360, mirrors MIL-PRF-2105E
and is a global standard used by oil companies in countries where U.S. military
standards are not applicable.
View Full Size Chart
Viscosity Grade (SAE J306)
A lubricant’s primary function is to reduce friction and wear, and its most important
property is its viscosity (thickness/resistance to flow). Lubricants are considered
incompressible and under ideal conditions maintain a constant layer of protection,
known as film strength, to keep moving parts from contacting each other. With no
direct contact, wear is eliminated. There is a point, however, at which heavy loads
exceed the oil’s ability to separate parts and metal-to-metal contact occurs. This
is, in part, a function of viscosity. The higher the viscosity of a lubricant, the
greater the load it can carry. Using gear lube that is too thick, however, has disadvantages.
Thicker oils are more difficult to circulate, particularly in cold temperatures,
and wear protection can be sacrificed. Thicker gear lubricants also require more
energy to circulate, which negatively impacts fuel economy. Additionally, thicker
gear lubes have higher internal resistance (intra-fluid friction) which causes them
to run hotter. There is no advantage to using a gear lube with a viscosity greater
than that required by the application. Conversely, gear lube that is too thin will
not have sufficient load-carrying ability to meet the equipment requirements.
The SAE has developed a grading system, SAE J306, which categorizes gear lubricants
based on their high- and low-temperature viscosities. An additional requirement
of SAE J306 is shear stability, which is explained later in this document. The viscosity
requirements for SAE 75W-90 gear lubricants are highlighted in green in the following
chart.
View Full Size Chart
Viscosity Index (ASTM D-2270)
Oil viscosity is affected by temperature changes during use. As a gear lubricant’s
temperature increases, its viscosity decreases, along with load-carrying ability.
The degree of change that occurs is determined by ASTM D-2270 and referred to as
the lubricant’s viscosity index (VI). ASTM D-2270 examines the viscosity change
that occurs between 40°C (104°F) and 100°C (212°F) . The higher the VI, the less
the viscosity changes with temperature. A high VI is desirable and, in part, indicates
higher lubricant quality. It does not, however, represent a lubricant’s high-temperature
viscosity or its load-carrying ability.
View Full Size Chart
Similar to 5W-30 automotive engine oils, 75W-90 gear lubricants are defined as multi-viscosity.
This means the gear lubricant has enough viscosity to protect against wear at high
temperatures, as well as good flow properties at cold temperatures. Many gear lubes
cannot fulfill both requirements without the use of VI improver additives. VI additives
keep lubricants from becoming too thick to flow in cold temperatures and too thin
to protect in high temperatures. VI additives have many uses. If used improperly
in gear lubricants, however, they can break down and lose viscosity through a process
called shearing. Because of this, the SAE incorporated the CEC L-45-A-99 (KRL) 20-Hour
Shear Test as a requirement for all automotive gear lubes. This specification requires
that gear lubes not shear down and fall below the minimum viscosity for that grade.
The KRL Test utilizes a tapered roller bearing and test cup filled with 40 ml. of
gear lube. The test parameters are set at 60°C (140°F), 1475 rpm, 5000 N load for
a duration of 1,740,000 motor revolutions (approximately 20 hours). Each gear lube’s
viscosity was recorded before and after the shear test as seen in the following
graph.
View Full Size Chart
This graph shows the initial oil viscosity and the viscosity after the shear test.
The SAE J306 high-temperature viscosity requirements (shaded area) for SAE 90 gear
lubes are between 13.5 centistokes (a unit of measure for viscosity designated as
cSt) and 18.49 cSt @ 100°C (212°F) maximum.
Lucas 75/90 Synthetic, at 22.35 cSt, and Royal Purple Max-Gear 75W-90, at 19.32
cSt, both exceed the maximum 18.49 cSt initial viscosity (red), failing the SAE
J306 requirements for SAE 90 gear lubricants. All other gear lubricants were within
the required high-temperature viscosity range prior to the KRL Shear Stability Test.
Viscosity measurements following the KRL Shear Stability Test revealed that seven
gear lubes sheared down below the minimum viscosity requirements (orange), failing
the shear stability requirements of the SAE J306. The two gear lubes with the largest
viscosity loss, as reflected in the following graph, were Royal Purple, losing 40.6%
of its viscosity, and Torco SGO Synthetic, losing 35.2% of its viscosity. Royal
Purple was the only gear lube to fail both the initial viscosity requirements and
the shear stability requirements. It started out too thick and ended up too thin.
Torco SGO Synthetic, which had the highest VI in the previous graph, finished the
shear stability test as the thinnest of all the oils at 9.97 cSt, far below the
minimum 13.5 cSt requirement. Lucas 75/90 Synthetic, with an initial viscosity that
exceeded the maximum requirements by 20.8%, passed the shear stability test, but
lost 34.5% of its viscosity, the third largest loss of viscosity. Both OEM gear
lubes, GM and Mopar, failed the minimum viscosity requirements after the shear test.
Of all the gear lubes tested, half did not meet the SAE J306 shear stability requirements.
AMSOIL Severe Gear 75W-90 was in the proper initial viscosity range and retained
the highest viscosity after the shear test with a viscosity of 16.03 cSt – the mid-point
of the SAE 90 viscosity grade.
View Full Size Chart
Low-Temperature Viscosity - Brookfield Viscosity Test (ASTM D-2983)
As temperature decreases, the viscosity of oil increases. Gear lubricants with high
viscosity at cold temperatures are less efficient, and the gears require more energy
to turn. Gears and bearings in the differential and axle housing are splashlubricated,
and gear lubricants that are too thick at cold temperatures can starve internal
components of lubrication, which can cause failure.
The cold-temperature viscosity of gear lubricants is indicated by the first number
in the SAE viscosity grade (75W of a 75W- 90 gear lube). The SAE J306 standard utilizes
the Brookfield Viscosity Test, recorded in centipoises (cP), to determine coldtemperature
performance. The maximum viscosity is 150,000 cP at the given temperature for the
SAE viscosity grade. For example, SAE 75W must be less than 150,000 cP at -40°C
(-40°F), while SAE 80W must be less than 150,000 cP at -26°C (-15°F).
In the Brookfield Viscosity Test, a glass test tube is filled with gear lube and
cooled to the appropriate temperature. A small spindle is inserted into the lubricant
and the maximum torque required to rotate the spindle is recorded. The torque reading
is used to calculate the viscosity in cP.
View Full Size Chart
Cold-temperature performance is impacted by a lubricant’s high-temperature viscosity.
High-viscosity gear lubes tend to have worse cold-temperature performance than low-viscosity
gear lubes. AMSOIL Severe Gear, however, with the highest after-shear viscosity,
exhibited the best cold-temperature properties of all gear lubes, except for Torco
SGO, which thinned out of grade in the shear test. Royal Purple and Lucas failed
the cold-temperature Brookfield requirements for 75W gear lubes, as well as the
high-temperature requirements for SAE 90 gear lubes, effectively disqualifying them
entirely from the SAE 75W-90 category. Royal Purple Max-Gear, having also failed
the Shear Stability Test, was the only gear lube to fail every parameter of the
SAE J306 requirements. Red Line was 14,100 cP over the maximum allowable viscosity
at 164,100 cP, and Castrol SYNTEC 75W-90 had a borderline pass at 149,850 cP. As
noted, SAE 80W-90 gear lubes are measured at -26°C (-15°F) and all test candidates
passed.
Red Line, Royal Purple and Lucas, having failed the viscosity requirement for SAE
75W, were then tested at the SAE 80W parameters for comparison purposes. Red Line
scored 18,250 cP and Royal Purple scored 24,700 cP, showing better performance than
the SAE 80W-90 gear lubes. Lucas, however, at 98,050 cP, showed worse cold-temperature
properties than Castrol 80W-90, which is reflected in the overall score on page
19.
Standard Pour Point Test Method (ASTM D-97)
Pour point can vary greatly depending on the construction of the product. Pour point and Brookfield viscosity both measure
the cold-temperature properties of gear lube, but are very different. Pour point is defined as the coldest temperature at
which oil will flow before solidifying. The Pour Point Test consists of a glass jar filled with gear lube which is cooled to a temperature
close to its pour point. The gear lube is checked at intervals of 3°C (5°F) for fluidity. When the gear lube no longer
flows, the pour point is recorded at the last temperature of fluidity.
View Full Size Chart
The SAE 80W-90 gear lube test results were between -26°C (-15°F) and -31°C (-24°F).
SAE 75W-90 gear lubes have better cold-temperature properties and therefore better
pour points. It is important to have a low pour point combined with a low Brookfield
viscosity value since it is possible to have a good low pour point but only a marginal
Brookfield viscosity. Castrol SYNTEC is a good example of this. SYNTEC had the best
pour point of the gear lubes tested, but a borderline Brookfield viscosity pass
at 149,850 cP. Lucas 75/90 Synthetic, on the other hand, did not perform well in
either area. It showed a pour point of -37°C (-35°F) and a Brookfield viscosity
of greater than 2,000,000 cP. AMSOIL Severe Gear 75W- 90 and Torco SGO Synthetic
had the best combined Brookfield and pour point scores.
Channel Point - Federal Test Method Standard (FTMS 791C) No. 3456
MIL-PRF-2105E is an extreme-pressure, hypoid gear lubricant specification established
by the U.S. military. It is more stringent than API GL-5. An additional requirement
of MIL-PRF-2105E is the lubricant’s ability to pass the Channel Point Test. While
not all gear lubes claim MIL-PRF-2105E, the channel point requirement is important
because channeling during coldtemperature operation may cause catastrophic gear
and bearing failure. A test sample container with 650 ml of oil is run through a
warming cycle before being placed in a temperature-controlled bath at -45°C (-49°F)
for SAE 75W-90 gear lubes and -35°C (-31°F) for SAE 80W-90 gear lubes. The test
is run for 18 hours +/- 2 hours. A groove 2 cm wide is then made in the gear lubricant
down to the bottom of the container. The gear lubes must completely fill the groove
and cover the bottom of the container in less than 10 seconds to pass the test.
Lucas 75/90 Synthetic and Valvoline High Performance 80W-90 were the only gear lubes
to fail the channel point test.
View Full Size Chart
High Temperature Oxidation Resistance Standard Test Method for Oxidation Characteristics
of Extreme-Pressure Lubrication Oils (ASTM D-2893 Method B)
Heat can destroy lubricants. High temperatures accelerate oxidation, which causes
acid development, corrosion, sludge and varnish deposits, lubricant thickening and
shortened gear lube life. Oxidized gear lubes lose lubricating effectiveness. Energy
efficiency goes down, wear goes up and cold-temperature flow properties are greatly
reduced. Heat and oxidation resistance are critical for proper gear lubrication
and long lubricant life.
ASTM D-2893 Method B test methodology measures the oxidation-resistance characteristics
of extreme-pressure lubricants. The test utilizes 41 mm x 600 mm test tubes filled
with 300 ml of gear lube, heated to 121°C (250°F) and aerated at 10 liters per hour.
The test is run for 312 hours (13 days). The gear lubes are then evaluated for viscosity
increase and precipitation of solids. Large increases in viscosity and deposit formation
indicate greater gear lube deterioration. In addition, 50 ml of each tested gear
lube was filtered through an 8 micron filter patch to show discoloration. Filtering
the lubricant for visual inspection is not a test requirement. The test parameters
simulate the severe conditions inside a differential.
View Full Size Chart
Solids Precipitation (measured in ml) All gear lubes measured at <0.05 ml with
the exception of Mopar Synthetic 75W-90 with Mopar LS additive, which measured at
0.08 ml, and Lucas 75/90 Synthetic, which measured at 0.25 ml.
Pennzoil Synthetic 75W-90 and AMSOIL Severe Gear 75W-90 had the best overall performance
in both categories, indicating high resistance to oxidation and extended lubricant
life. Pennzoil Synthetic 75W-90 showed the lowest viscosity increase, and AMSOIL
Severe Gear had the cleanest high-temperature deposit properties. While petroleum-based
Pennzoil Gearplus 80W-90 and Lucas 75/90 Synthetic showed limited viscosity increase,
they both left significant deposits on the filter patches. Castrol SYNTEC 75W-90
thickened by 16.45%, yet had clean performance...
Wear Reduction
In automotive differentials the ring and pinion are spiral-cut, hypoid gears. They
slide more on each other than other types of gears. Although spiral-cut gears allow
for quieter operation, under load their extreme sliding action can wipe the lubricant
film from between the gears. High levels of extreme-pressure additives are used
to protect these gears when the lubricant film is wiped away or ruptured.
Many different tests are used to measure the extreme-pressure and anti-wear performance
of lubricants. Three ASTM laboratory tests were selected that operate under different
extreme-pressure and anti-wear conditions. These tests include the 4-Ball Extreme
Pressure Test, the Falex Pin and V-Block Test and the 4-Ball Wear Test. Good performance
in all of the tests indicates good anti-wear and extreme-pressure protection.
Extreme-Pressure (EP) Property Measurements (4-Ball EP Test ASTM D-2783)
The 4-Ball Extreme-Pressure Test evaluates extreme-pressure properties and high-load,
anti-wear protection properties. High reported values indicate the gear lube provides
better protection against wear and galling when the lubricant film is ruptured under
heavy loads. Towing, hauling, racing and high-horsepower/torque applications are
examples of severe service where the lubricant film is commonly ruptured and metal-to-metal
contact occurs.
The 4-Ball EP Test is operated with one steel ball under load rotating at 1760 rpm
against three steel balls submerged in oil and held stationary in a cradle. The
temperature of the gear lube is brought to 18.33 to 35.0°C (65 to 95°F).Weld point
and load-wear index are determined from a series of 4-Ball EP Test runs.
Weld Point
A series of tests with increasing loads, measured in kilograms (kg) are performed
until the fourth loaded ball seizes (welds) to the three stationary balls. The weld
point is the lowest (first) extreme-pressure point which exceeds the lubricant’s
loadcarrying ability. It is a good indicator of a lubricant’s extreme-pressure properties.
Gear lubes with weld points of 400kg indicate better EP properties than those with
weld points of 315kg.
View Full Size Chart
It should be noted that good performance in one test does not necessarily mean good
performance in both tests. An example of mixed performance is Lucas 75/90 Synthetic,
which had a high load-wear index value but a low score in the weld parameter. Pennzoil
Synthetic 75W-90 had low scores in both test parameters.
Extreme-Pressure Property Measurements Falex Pin and V-Block One Minute Step Test
(ASTM D-3233B)
The Falex Extreme Pressure Test differentiates between lubricants having low, medium
and high levels of extreme-pressure properties by measuring their load-carrying
capacities. The Falex Test consists of a steel pin that rotates at 290 rpm against
two stationary V-blocks in 250-lb. increments. Each 250-lb. increment is applied
for 60 seconds and failure is recorded when either the pin seizes to the V-blocks
or the wear between the pin and V-blocks is so rapid that the loading gear cannot
keep the applied load constant.
View Full Size Chart
High numerical values represent better extreme-pressure properties. Six of the gear
lubes scored 2500 lbf (pounds force) or greater, indicating a higher level of protection
compared to the remaining eight lubricants. When looking at the combined Falex Test
results and 4-Ball EP Test results, it is noted that all three petroleum SAE 80W-90
gear lubes consistently scored lower than most of the other oils. When evaluating
the top six gear lubes in the Falex Test, only AMSOIL, Red Line and Mobil placed
in the top six in both categories of the 4-Ball EP Test, ahead of GM, Lucas and
Valvoline which had good 4-ball EP load-wear index scores.
Wear Preventative Characteristics of Lubricants (4-Ball Wear Test ASTM D-4172)
This test evaluates the anti-wear properties of fluid lubricants in sliding contact
and under lighter loads than those used in the 4-Ball EP Test. It is conducted using
the 4-Ball Anti-Wear Test procedure and measurements, which are different than the
4-Ball EP Test procedure. The standard test parameters of the 4-Ball Wear Test are
75°C (167°F), 40kg load, 1200 rpm for 1 hour. The wear scar diameter of the three
stationary balls is measured and the average is reported as the wear scar in mm.
View Full Size Chart
The anti-wear 4-Ball Test results were much closer than in the higher-loaded tests.
Note: Although in some cases the test results are negligible between oils, results
were taken as recorded for scoring purposes. There are, however, some interesting
observations when comparing the data to the other extreme-pressure and anti-wear
testing. Pennzoil Synthetic 75W-90 scored worst in the 4-Ball EP LWI, yet scored
best in the 4-Ball Wear Test. And out of all the extreme-pressure and anti-wear
testing, AMSOIL Severe Gear consistently scored in the top four in all categories,
which indicates that the AMSOIL lubricant offers superior protection under widely
varying operating conditions. Examples of wear scars from one of the three stationary
balls from 4-ball wear testing performed on AMSOIL Severe Gear 75W-90 and Lucas
75/90 synthetic gear lubes are shown below.
Foam Resistance
During differential operation, gears and bearings turn at high speeds which churn
the lubricant. When air is introduced, foaming can occur. While gear lube is considered
incompressible, air is compressible and when bubbles pass between loaded areas,
the bubbles collapse and metal-to-metal contact occurs, causing wear. Foam can also
increase friction and act as an insulator, which increases heat and oxidation. Good
foam control is important in gear lubricants. In most cases, anti-foam additives
are needed. The API has established maximum foam limits for GL-5 gear lubricants.
Foaming Tendencies (ASTM D-892)
This test measures the foaming characteristics of lubricating oils. It consists
of a 1,000-ml graduated cylinder fitted with an air diffuser in the bottom. The
cylinder is filled with 190 ml of gear lube and heated to 24°C (75°F) (Sequence
I).The air passing through the diffuser is adjusted to 94ml/min and percolates up
through the test lubricant. The test is run for five minutes and the air is shut
off. Any foam that forms on the surface of the lubricant is then measured. After
10 minutes of settling time, foam levels are measured again. The procedure is repeated
for Sequence II with 180 ml of lubricant at 93.5°C (200°F), then back down to 24°C
(75°F) and 190 ml of lubricant for Sequence III. The test results are reported as
x/x for each of the three sequences; the first number indicates foam immediately
after the test, and the second number indicates foam after settling. In addition
to testing fresh gear lubes, testing was done on the “aged” gear lubes after oxidation
testing.
Example of Non-Foaming Oil
Oxidation can change a lubricant’s properties and negatively impact foam performance.
Note that API GL-5 does not require a foam test on aged, oxidized oils. This was
done strictly to simulate in-service operation.
The API GL-5 specification has established a maximum limit of 20/0 in Sequence I,
50/0 in Sequence II and 20/0 in Sequence III.
Gear lubes failing the GL-5 requirements are marked in red under the New Oil heading.
Gear lubes in the right column report foam results on the aged, oxidized oils. GM
Synthetic 75W-90 passed the API GL-5 requirement but generated significant amounts
of foam after oxidation. Pennzoil Synthetic 75W-90 and Lucas 75/90 Synthetic, on
the other hand, failed the initial API GL-5 requirement but passed after oxidation
testing.
View Full Size Chart
Copper Corrosion Resistance
Extreme-pressure additives in gear lubricants become more chemically active when
subjected to heat. Copper and brass are soft metals and are subject to attack from
acids, sulfur compounds and other chemicals in gear lubricants. When corrosion attacks
these components it can be seen as a discoloration and occasionally forms buildup
on the surface of the component. Acidic corrosion results in wear, which can lead
to component failure.
Copper Corrosion (ASTM D-130)
The standard Copper Corrosion Test is designed to assess the corrosive characteristics
of lubricants. In this test a polished copper strip is immersed in a test tube with
a given quantity of sample fluid. The entire test tube is then immersed into a bath
which is heated to either 100°C (212°F) or 121°C (250°F) for three hours. The hotter
temperature is more severe. The copper strip is then removed, washed and evaluated
according to ASTM Copper Strip Corrosion Standards (shown below). Corrosion is evident
from discoloration. The test results are reported in a range from 1a to 4c. API
GL-5, MT-1 and MIL-PRF-2105E all require the hotter 121°C (250°F) test temperature.
However, API MT-1 and MILPRF- 2105E have a tighter specification limit for a pass,
requiring 2a as opposed to 3a for GL-5.
View Full Size Chart
To determine test results, a technician rates the components by comparing them to
the copper corrosion standard. Mopar 75W-90 and Royal Purple Max-Gear 75W-90 each
displayed black streaks and received 4a ratings. Lucas 75/90 Synthetic was clearly
corroded and was given a 4b rating.
Pricing
The price of a product is most often a consumer’s first concern when selecting a
gear lube. Price, however, does not reflect the actual cost of a product. Less expensive
oils may save money initially, but may cost more in the end if the products compromise
performance or require more frequent oil changes. Ford, for example, requires petroleum
gear lubes to be changed every 3,000 miles under severe service but waives that
requirement for synthetic gear lubes, extending the service life.6 In this study,
the three lower-priced petroleum SAE 80W-90 gear lubes had consistently lower test
scores in the 4-Ball EP and Falex Tests. Generally, lower performance is associated
with lower price. There are, however, exceptions. Lucas 75/90 Synthetic and Royal
Purple Max-Gear Synthetic 75W-90 demonstrated that price is not necessarily consistent
with performance.
The benefits provided by a well-engineered, although higher-priced gear lube, can
easily offset that higher price. Paying a little more for a quality lube that delivers
the right performance is a low-cost investment to protect high-priced equipment.
In this study, the pricing was obtained by purchasing a 12-quart case of each product
from the manufacturers or distributors and calculating the cost per quart.
Scoring and Summary of Results
Each gear lubricant was assigned a score for each test result. The gear lube with
the best test result was assigned a 1. The gear lube with the second best result
was assigned a 2, and so on. If two or more oils tied, the next best score was ranked
according to the number of oils preceding it. In pass/fail testing, passing gear
lubes were given scores of 1. Failing gear lubes were given scores dependent on
the number of gear lubes that passed. For example, of the 14 gear lubes tested,
12 passed the Channel Point Test and received scores of 1. With 12 gear lubes passing,
the two failing gear lubes received scores of 13. In the Solids Precipitation evaluation,
differentiation was immeasable in the 12 gear lubes with values of .05 ml. Differentiation
was measurable in those gear lubes that scored greater than .05 ml, and they received
scores of 13 and 14 respectively. Note that the results of each test have not been
weighted to suggest the degree of significance it represents. The degree of significance
is left to the consumer to decide. The results in all categories were added to produce
an overall total for each gear lube. The gear lube with the lowest total demonstrated
the best overall performance. Red scores did not meet either API GL-5 performance
requirements or SAE J306 viscosity requirements.
Conclusion
As the testing indicates, AMSOIL Severe Gear ranked highest among all gear lubes
tested. It was the only gear lube to score a 4 or better in all performance categories.
The high ranking of AMSOIL Severe Gear clearly points to a well-balanced formulation
capable of delivering effective, long-lasting lubrication protection to all differential
components. Most notable is the superior performance of AMSOIL Severe Gear in the
critical areas of extreme-pressure protection and viscosity and oxidation stability.
Based on the performance testing, the slightly higher than average price of AMSOIL
Severe Gear would be offset by the cost savings achieved through reduced maintenance,
longer lasting differentials and extended lubricant life.
Some gear lubes tested well in some areas but scored low marks in others. Torco
SGO Synthetic scored highest in viscosity index and cold-temperature Brookfield
viscosity but sheared out of grade, failing the SAE J306 requirements for SAE 75W-90
gear lubes. Mopar and Royal Purple scored well in the 4-Ball EP Weld Test, but failed
the Copper Corrosion Test and GM, with a good 4-ball EP score, foamed badly after
oxidation. This would indicate that too much emphasis in one area of formulation
can detract from performance in others. A gear lube is only as good as its weakest
link.
A well-balanced gear lube formulation, therefore, is critical for differentials
in all types of vehicles, both standard and highperformance. With more horsepower,
more towing capacity, higher hauling limits and changes in vehicle design, more
stress than ever is placed on differential gears. High-quality lubrication is essential,
and awareness is now necessary to ensure maximum differential performance and to
avoid costly repairs. When purchasing gear lube the decision is left to the consumer,
yet based on the facts reported in this document, AMSOIL Severe Gear is the logical
choice.
Note: To further verify the findings, additional testing was performed on AMSOIL
Severe Gear. The L-37 Axle Rig Test evaluates load-carrying, wear protection and
extreme-pressure properties of gear lubricants. The severity of the test was increased
to challenge AMSOIL Severe Gear to the absolute limits in gear lube performance.
See Appendix C for test parameters and results.
Sources:
1. Richardson, Robert; Marsic, Vera; Tarrant, Simon: “Driveline Fluids - Thermal
Management Challenges and Impacts on Base Oil and Additive Technologies,” National
Petrochemical & Refiners Association Annual Meeting, Paper AM-05-32, March,
2005. 2. 2007
2. Trailer Life Towing Guide.
3. O’Conner, B.M.; Schenkenberger, C.: “The Effect of Heavy Loads on Light Duty
Vehicle Axle Operating Temperature,” Powertrain & Fluid Systems Conference and
Exhibition, SAE Paper #2005-01-3893, October, 2005.
4. Tocci, Lisa: “Torque Spark.” Lubes ‘N’ Greases, September 2007.
5. Mitchell Repair Information Company, LLC.
6. Motor Information Systems Check Chart 2007 Quick Lubrication Guide.
Additional Sources: Lubrizol website.
Lubrizol Ready Reference Manual for Lubricants and Fuels, 2005.
Download the Complete White Paper in PDF Format