A Study of Automotive Gear Lubes – Part 1

AMSOIL Drivetrain Division
September 2007


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.


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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.


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.


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.

Brand API MIL-L-2105D MIL-PRF-2105E/
SAE J2360
Batch Number
GL-5 MT-1
Synthetic Candidates
AMSOIL® Severe Gear® 75W-90 X X X LN 25902
Castrol® SYNTEC® 75W-90 X X MD62597BTW2578
GM® Synthetic Axle 75W-90 X 101006
Lucas® 75/90 Synthetic X X 193
Mobil 1® Synthetic 75W-90 X OEV5J6CO1788#5990
Mopar® Synthetic 75W-90 with Mopar® LS additive X MRT081205C
Pennzoil® Synthetic 75W-90 X Unreadable
Red Line® Synthetic 75W-90 X X X 56292/60840001663
Royal Purple® Max-Gear® 75W-90 X X ICPH01606
Torco® SGO Synthetic 75W-90 with Torco® Type G LS additive X L46680 LNRAGMO
Valvoline® SynPower® 75W-90 X X X F076C
Petroleum Candidates
Castrol® Hypoy C 80W-90 X X PO61165
Pennzoil® Gearplus® 80W-90 X JDSC 26615512155
Valvoline® High Performance 80W-90 X X X C1860

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.

Automotive Gear Lubricant Viscosity Classifications – SAE J306 – June 14, 2005
SAE Viscosity Grade Max. Temperature for Viscosity of Kinematic Viscosity at 100°C (cSt)3
150,000 cP (°C)1,2 Min.4 Max.
70W -55 5 4.1 -
75W -40 4.1 -
80W -26 7.0 -
85W -12 11.0 -
80 - 7.0 <11.0
85 - 11.0 <13.5
90 - 13.5 <18.5
110 - 18.5 <24.0
140 - 24.0 <32.5
190 - 32.5 <41.0
250 - 41.0 -

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.

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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.


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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.


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Continue to Part 2….

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