The most important internationally accepted classification system defining automotive gear oil performance characteristics is laid down by the American Petroleum Institute (API)
Designates the type of service characteristics of automotive spiral –bevel worm gear axles and some manually-operated transmissions operating under such mild conditions of low unit pressures and sliding velocities that straight mineral oil can be used satisfactorily. Oxidation and rust inhibitors, anti-foam additives and pour point depressants may be outclassed to improve the characteristics of lubricants for this service. Does not contain friction modifiers and extreme pressure agents.
Designates the type of service characteristics of automotive-type worm-gear axles operating under such conditions of load, temperatureand sliding velocities that lubricants satisfactory for API GL-1 service will not suffice. Products suited for this type of service contain anti-wear or very mild extreme pressure agents which provide protection for worm gears.
Designates the type of service characteristics of manual transmissions and spiral-bevel axles operating under moderately severe conditions of speed andr load. These service conditions require a lubricant having a load-carrying capacity greater than that which will satisfy API GL-1 but below the requirements of lubricants satisfying API GL-4 service
Designates the type of service characteristics of gears operating under severe conditions of sliding speed, particularly hypoid , in passenger cars and other automotive-type equipment operated under high-speed/low-torque and low-speed/high-torque conditions.
Designates the type of service characteristics of gears, particularly hypoid, in passenger cars and other automotive equipment operated under high-speed/shock-loaded, high-speed/low-torque and low-speed/high-torque conditions.
This is an obsolete classification and listed for historical reference only. The equipment used to measure performance is no longer available.