The most important internationally accepted classification system defining automotive gear oil performance characteristics is laid down by the American Petroleum Institute (API)
API GL-1
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.
API GL-2
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.
API GL-3
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
API GL-4
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.
API GL-5
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.
API GL-6
This is an obsolete classification and listed for historical reference only. The equipment used to measure performance is no longer available.