The American Petroleum Institute (API) service designations are based on the type of service in which components will be used. The designations are utilized by manufacturers to select lubricants for particular gear types and operating conditions. No attempt is made by the API classification system to classify gear oils by physical properties or test performance. It also recognizes that some lubricants are suitable for a wide range of operating conditions and may be recommended for more than one service designation.

Although API designations may be very useful when making general recommendations, manufacturer recommendations should always be consulted to ensure that the lubricant being considered is not prohibited by that manufacturer.

API-GL-1 designates the type of service characteristics of automobile spiral bevel and worm gear axles as well as some manually operated transmissions operating under such mild conditions of low unit pressures and sliding velocities in which straight mineral oil can be used satisfactorily. Oxidation and rust inhibitors, defoamers and pour-point depressants may be utilized to improve the characteristics of lubricants for this service. Frictional modifiers and extreme pressure (EP) agents are not used. This designation is recommended for use in some manual truck transmissions.

API-GL-2 refers to the type of service characteristics of automotive type worm gear axles operating beyond GL-1. It may contain anti-wear and very mild EP agents, and usually includes fatty additives for worm gears. This service designation is obsolete.

API-GL-3 describes the type of service characteristic of manual transmissions and spiral bevel axles operating under moderately severe conditions of speed and load beyond GL-2 but below GL-4. It may have mild EP agents but is not intended for hypoid gearing. This service designation is obsolete.

API-GL-4 relates to the type of service characteristics of gears, particularly hypoid gears operated under non-critical, moderate speed, shock load; high speed, low torque; and low speed, high torque conditions.

API-GL-5 designates the type of service characteristics of gears, particularly hypoid gears in passenger cars and other automotive equipment operated under high speed, shock load; high speed, low torque; and low speed, high torque conditions. This designation is still widely used for EP gear oils.

API-GL-6 is associated with oils that reduce gear scuffing in older high-performance cars. This service designation is obsolete.

API-MT-1 describes a high EP oil intended for some non-synchronized manual truck and bus transmissions.

Mack and Volvo trucks (as well as others) have their own specifications that address some conditions beyond GL-5. These are Mack GO-J+ and Volvo 97310, although these numbers change every few years.