The API (American Petroleum Institute) defines automotive gear lubricant service designations to assist manufacturers and users of automotive equipment in the selection of transmission, transaxle and axle lubricants based on gear design and operating conditions.
Selecting a lubricant for specific applications involves careful consideration of the operating conditions and the chemical and physical characteristics of the lubricant. The API designations also recognize the possibility that lubricants may be developed for more than one service classification.
Gear oils are classified by the API using the letters GL (abbreviation for Gear Lubricant) followed by a number to identify the performance level of the oil. The API has also issued the MT-1 service designation for certain non-synchronised manual transmissions. Only three of the seven automotive gear lubricant service designations issued by the API are currently in use due to changes in manufacturers’ recommended practices or due to the unavailability of testing hardware.
The API Lubricant Service Designations for Automotive Manual Transmissions, Manual Transaxles, and Axles are described below, followed in some instances by supplemental comments (in italics) regarding the use of these lubricants:
API GL-1 (Obsolete)
This designation denotes lubricants intended for manual transmissions operating under such mild conditions that straight petroleum or refined petroleum oil may be used satisfactorily. Oxidation and rust inhibitors, antifoam agents and pour depressants may be added to improve the characteristics of these lubricants. Friction modifiers and extreme pressure additives shall not be used.
API GL-1 lubricants are generally not suitable for most passenger car manual transmissions. However, these
oils may be used satisfactorily in some truck and tractor manual transmissions. Lubricants meeting service designation API MT-1 are an upgrade in performance over lubricants meeting API GL-1 and are preferred by commercial vehicle manual transmission manufacturers.
API GL-2 (Obsolete)
The designation API GL-2 denotes lubricants intended for automotive worm-gear axles operating under such conditions of load, temperature, and sliding velocities that lubricants satisfactory for API GL-1 service will not suffice. Products suited for this type of service contain anti-wear or film-strength improvers specifically designed to protect worm gears.
API GL-3 (Obsolete)
This designation denotes lubricants intended for manual transmissions operating under moderate to
severe conditions and spiral-bevel axles operating under mild to moderate conditions of speed and load. These
service conditions require a lubricant having load-carrying capacities exceeding those satisfying API GL-1 service
but below the requirements of lubricants satisfying API GL-4 service.
Gear lubricants designated for API GL-3 service are not intended for axles with hypoid gears. Some transmission
and axle manufacturers specify engine oils for this service.
API GL-4 (Current)
The designation API GL-4 denotes lubricants intended for axles with spiral bevel gears operating under moderate
to severe conditions of speed and load, or axles with hypoid gears operating under moderate conditions of speed
and load. Axles equipped with limited-slip differentials have additional frictional requirements that are normally
defined by the axle manufacturer.
API GL-4 oils may be used in selected manual transmission and transaxle applications where API MT-1 lubricants
are unsuitable. In all cases, the equipment manufacturer’s specific lubricant quality recommendations should be
followed.
API GL-5 (Current)
This designation denotes lubricants intended for gears, particularly hypoid gears, in axles operating
under various combinations of high-speed/shock load and low-speed/high-torque conditions. Frictional requirements for axles equipped with limited-slip differentials are normally defined by the axle manufacturer.
API GL-6 (Obsolete)
The designation API GL-6 denotes lubricants intended for gears designed with a very high pinion offset. Such
designs typically require protection from gear scoring in excess of that provided by API GL-5 gear oils.
A shift to more modest pinion offsets and the obsolescence of original API GL-6 test equipment and procedures have eliminated the commercial use of API GL-6 gear lubricants.
API MT-1 (Current)
This designation denotes lubricants intended for non-synchronized manual transmissions used in buses
and heavy-duty trucks. Lubricants meeting the requirements of API MT-1 service provide protection against the
combination of thermal degradation, component wear, and oil-seal deterioration, which is not provided by lubricants in current use meeting only the requirements of API GL-4 or GL-5.
API MT-1 does not address the performance requirements of synchronized transmissions and transaxles in
passenger cars and heavy-duty applications.
Automatic or semi-automatic transmissions, fluid couplings, torque converters, and tractor transmissions usually require special lubricants. Consult the equipment manufacturer or your lubricant supplier for the proper lubricant for these applications.
The API Automotive Gear Oil Classifications only specify performance level and service designation. Viscosity limits for automotive gear lubricants are described by the SAE J306 standard as discussed in OilChat #4.