Axle Oil Specifications: from API GL-1 to Today

There are many specifications surrounding automotive gear lubricants. The most relevant axle oil standards are issued by the vehicle and equipment manufacturers, ASTM International, the American Petroleum Institute (API), and the Society for Automotive Engineers (SAE); API and SAE standards are discussed below.

API Categories GL-1, GL-2, GL-3 and GL-6 (marked with * below) were declared inactive by SAE Technical Committee 3 in 1995, even though oils may be marketed with these designations. ASTM does not plan to maintain the performance tests associated with these categories, as in a number of cases these tests can no longer be run because parts or test installations are not available.

The performance requirements for automotive gear lubricants depend on their intended use.

Category Status Used In Operating Conditions Notes
API GL-1 Inactive* manual transmissions mild conditions: low unit pressures and minimum sliding velocities Oxidation and rust inhibitors, defoamers and pour depressants may be used to improve the characteristics of lubricants intended for this service. Friction modifiers and extreme pressure additives shall not be used.
API GL-2 Inactive* automotive type worm-gear axles conditions of load, temperature and sliding velocities that lubricants satisfactory for API GL-1 service will not suffice
API GL-3 Inactive* manual transmissions and spiral-bevel axles mild to moderate to severe conditions of speed and load Load-carrying capacities greater than APL GL-1, but below the requirements of lubricants satisfying the API GL-4 service.
API GL-4 Active spiral-bevel and hypoid gears in automotive axles moderate speeds and loads These oils may be used in selected manual transmission and transaxle applications.
API GL-5 Active gears, particularly hypoids in automotive axles high-speed and/or low-speed, high-torque conditions Lubricants qualified under U.S. Military specification MIL-L-2105D (formerly MIL-L-2105C), MIL-PRF-2105E and SAE J2360 satisfy the requirements of the API GL-5 service designation.
API GL-6 Inactive* gears designed with a very high pinion offset designs typically require (gear) score protection in excess of that provided by API GL-5 gear oils The original API GL-6 test equipment is obsolete.
API MT-1 Active non-synchronized manual transmissions used in buses and heavy-duty trucks protection against the combination of thermal degradation, component wear, and oil seal deterioration Exceeds protection provided by lubricants meeting only the requirements of API GL-4 and API GL-5.
MIL-PRF-2105E Re-written as SAE Standard J2360 axles combines performance requirements of predecessor (MIL-L-2105D) and API MT-1 Requirements: chemical/physical, stationary axle test, field test, data review by Lubricants Review Institute, and from API MT-1: stringent oil seal compatibility and thermal durability test requirements.
SAE J2360 Active performance equivalent to that defined by MIL-PRF-2105E It includes all of the most recent axle and transmission testing requirements identified in API GL-5, API MT-1, and MIL-PRF-2105E including the need to demonstrate proof-of-performance through rigorous field testing.