Information materials:

API manual transmission oil grades

API Service category classes for lubricants for automotive transmissions and axle

This information was prepared by the Lubricant Classification Working Group of the API (American Petroleum Institute) Department of Marketing’s Committee on Fuels and Lubricants to assist manufacturers and users in selecting transmission oils.

Transmissions, especially rear axles, have a variety of gear designs available for a variety of operating conditions. Selecting oils for specific applications involves careful consideration of operating conditions and the chemical and physical characteristics of the oils to meet those conditions. Until recently, gear and differential gear oils were described in qualitative terms and by various names, each attempting to describe both the oils and the operating conditions under which the oil should perform. Keeping the designations of these lubricants to a minimum was considered very desirable considering current technology. Thus, the API has prepared seven designations for the service categories of oils for automotive manual transmissions and axles. Each designation refers to the characteristics required of a gear oil for a particular type of automotive service. These designations also recognize the possibility that a lubricant may be designed for more than one service category and, consequently, may be so designated.

In developing the language for the classification, the need was recognized to supplement the descriptions for certain gear lubricants, particularly for hypoid gears, by referring to a series of tests that are “test language” to provide more detailed information on the performance requirements of such oil. This “test language” was developed by Section III of Technical Division B Automotive Lubricants of ASTM Committee D-2, and references to these test procedures are made in the API category designations described below.

Lubricants Classifications For Manual Transmissions

Category Service
API GL-1 Refers to the type of operating conditions characteristic of automotive spiral bevel and worm gears and certain manual transmissions that operate under such mild conditions of specific pressures and sliding speeds that they can be satisfied by the use of a pure mineral oil. Oxidation and rust inhibitors, defoamers and depressant additives can be used to improve the performance of lubricants in this category. Friction modifiers and extreme pressure additives are not used.
API GL-2 Refers to the type of operating conditions characteristic of automotive worm gears operating under conditions of temperature, load and sliding speed that cannot be met by API GL-1 oils.
API GL-3 Refers to the type of operating conditions typical of manual transmissions and helical bevel axles operating under moderately severe speed and load conditions. These operating conditions require the lubricant to have a load carrying capacity greater than that which meets the requirements of API GL-1, but lower than the requirements for lubricants meeting API GL-4.
API GL-4 Refers to the type of operating conditions characteristic of manual transmissions, particularly hypoid* transmissions in passenger cars and other automotive equipment that operate in high-speed, low-torque and low-speed, high-torque applications.

Oils that provide anti-scuffing protection equal to or better than that defined by CRC Reference Gear Oil RGO-105 and that have been tested and provide the performance levels described in ASTM STP-512A of March 1987** are eligible for this category.

This classification category is still used commercially to describe lubricants, but the equipment required for the test procedures to evaluate the effectiveness of oils is no longer available.

API GL-5 Refers to the type of operating conditions characteristic of gears, particularly hypoid passenger cars and other automotive equipment operating at high speeds and shock loads; at high speeds with low torque; at low speeds with high torque.

Suitable for this category are oils that provide anti-scuffing protection equal to or better than that specified in CRC Reference Gear Oil RGO-110, and that have been tested and provide the level of performance described in ASTM D7450*.

API GL-6 This category is out of date and is listed for historical reference only.

Equipment used to measure performance is no longer available.

API MT-1 Indicates the type of operating conditions specific to non-synchronized manual transmissions used in buses and heavy-duty vehicles. Oils meeting the requirements of the API MT-1 category provide protection against a combination of thermal degradation, component wear and seal damage that modern oils meeting only API GL-4 or API GL-5 requirements do not provide.

API MT-1 oils do not meet the performance requirements for synchronized transmissions and transmissions in passenger cars and heavy vehicles. API MT-1 oils must not be mixed with engine oils in the same gearbox. Transmission manufacturer’s oil quality recommendations must be adhered to.

API MT-1 specifications are defined in the latest version of ASTM D5760.

*  Limited slip differentials usually have special oil requirements. The oil supplier should be consulted as to its suitability for such differentials. Information useful in evaluating lubricants for this type of gear can be found in ASTM D7450.

** The full title of the publication is «Laboratory Testing of API GL-5 Automotive Gear Lubricants».


API GL-4 oils are not suitable for heavily loaded hypoid gears. API GL-4 oils are used in synchronized manual transmissions, lightly loaded hypoid and spiral bevel gears.

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