Transmission oil specifications

Not every transmission oil has the same composition, because every transmission has different demands on the lubricant. For this reason, transmission oils are divided into different classes or provided with OEM approvals, similar to the specifications for engine oils. This makes it easy to determine which transmission oil can be used for which vehicle. You are also welcome to use our Oilfinder for your search. Typical classifications for transmission oils are based on viscosity (SAE classes), API specification or OEM approval.

Viscosity of transmission oils - SAE classes

The appropriate viscosity of a transmission oil contributes significantly to maintaining the required lubricating film thickness between the gear components. It is a simplified description of the internal resistance of an oil to flow. For driveline fluids it is classified worldwide into viscosity classes according to the US-American standard SAE J306 (Society of Automotive Engineers). There are viscosity classes from SAE 65 to SAE 250. The classification is based on the kinematic viscosity (measured in mm²/s) at 100°C. The higher the number, the thicker the oil.

Similar to engine oils, there are also viscosity classes for low-temperature applications, marked with the letter "W" from 70W to 85W (capable of flowing at temperatures down to -55°C or -12°C respectively). These "W" or winter classes classify the oils according to their flowability at low temperatures. Classification is based on the temperature at which the dynamic viscosity measured with a Brookfield viscometer does not exceed 150,000 mPa*s. Consequently, the Brookfield value of a SAE 80W automotive transmission oil at -26°C must not exceed 150,000 millipascal seconds.

SAE class	Max. Brookfield viscosity of 150,000 mPas at temperature in °C	Min. kinematic viscosity at 100°C in mm²/s	Max. kinematic viscosity at 100 °C in mm²/s
70W	-55	3.8	-
75W	-40	3.8	-
80W	-26	8.5	-
85W	-12	11.0	-
65	-	3.8	< 5.0
70	-	5.0	< 6.5
75	-	6.5	< 8.5
80	-	8.5	< 11.0
85	-	11.0	< 13.5
90	-	13.5	< 18.5
110	-	18.5	< 24.0
140	-	24.0	< 32.5
190	-	32.5	< 41.0
250	-	41.0	-

For multigrade oils such as 80W-90, the "80W" describes the low temperature properties, the "90" the thickness of the oil. The limit values of both classes have to be met. Transmission oils with SAE 80W-90 and 85W-90, which are still widespread, are mainly produced from mineral base oils. For thicker oils such as 80W-140 an additional shear-stable viscosity index improver is needed to meet the cold flow requirements down to -26°C. For even lower temperature levels with 75W or 70W at a viscosity level of SAE 90 or higher, the formulation effort will increase rapidly. For this purpose, synthetic base oils according to API group III, IV or V are required to guarantee the flowability of automotive transmission oils even at temperatures of -55°C and below.

The choice of the viscosity depends on the requirements on an optimized lubricating film thickness for the respective transmission application. If the oil is too thick, the gears must work against the viscosity. This results in higher splash losses, which reduce the efficiency, and higher temperatures, which cause the oil to age faster. However, if the viscosity is too low, the lubricating film thickness required to separate the friction partners cannot be achieved. The outcome is a higher risk of damage due to faster wear under mixed friction conditions. Which viscosity is the most suitable for your transmission is specified by the respective vehicle manufacturer. Follow the instructions in your operating manual or rely on the advice of our application technology experts.

Classification of transmission oils according to API

The American Petroleum Institute (API) does not only define international specifications for engine oils. With the classification of automotive transmission oils in six GL classes (Gear Lubricant), the API has specified the most common classification system. It defines the corresponding basic requirements depending on the field of application. Depending on the predominant load regime in the gearbox, a different composition of additives will be required.

API class	Application	Composition
GL-1 (inactive)	Oils for gearboxes with low contact pressure and low sliding speeds	Oils w/o additives were usually sufficient, antioxidants, corrosion protection additives, defoamers or pour point reducers can be used. Friction modifiers or EP additives are not allowed.
GL-2 (inactive)	Oils in worm gears for axles vehicles that are not covered by API GL 1 with regard to load, temperature and sliding speeds.	Oils with wear-reducing additives, but without EP additive requirements
GL-3 (inactive)	Oils for manual gearboxes and bevel transmissions in trucks with light to medium speed and load requirements.	Oils with EP additives for higher load carrying capacity than API GL-1 oils, but below requirements for API GL-4
GL-4	Oils for bevel transmissions and hypoid gearboxes in axles under light load and speed conditions as well as for selected shift and transaxle gearboxes	Oils with EP additive content above GL-3 level and additional requirements on corrosion protection
GL-5	Oils especially for hypoid gears (differential) in axle drives at high speed and/or low speed and high torque conditions	Oils with high content of EP additives and requirements on corrosion protection and ageing stability
GL-6 (inactive)	Oils for gearboxes with very large pinion offset (the design has not been accepted - performance tests required for API GL-6 are no longer available)	Oils with very high EP additives beyond the API GL-5 dimension
MT-1	Oils for non-synchronized manual transmissions in buses and trucks	Oils with high requirements on thermal stability, sealing compatibility, and higher corrosion and wear protection than oils for API GL‑5

The API classes GL-1, GL-2, GL-3 and GL-6 were rated "inactive" in 1995. However, transmission oils can still be marketed with these specifications.

Important note for mixing transmission oils:

It is not recommended to mix transmission oils of different API classes with each other. Manufacturer information should also not be ignored. If your car needs an API GL-5 oil, no GL‑3 or GL-4 oil should be added. GL-3/4 oils with their additives will not keep up with the high loads and cannot prevent accelerated wear. If you otherwise fill a GL-5 oil into a manual gearbox that is designed for GL-3 or GL-4, the usually lower friction will lead to slippage of the synchronizer in the gearbox. In the worst-case scenario, the whole circuit will fail.

OEM approvals for transmission oils

Classification according to SAE and API is only a simplified basis. The actual performance of transmission oils is determined by meeting additional specifications directly specified by transmission or vehicle manufacturers (OEMs = Original Equipment Manufacturers). OEMs define requirements for specific transmission or vehicle types of a brand. When purchasing transmission oils, therefore, pay attention to the required specifications or approvals. Follow the instructions in your operating manual.

Important OEM performance classes of transmission oils are:

For vehicles with automatic transmissions, General Motors and Ford are leading manufacturers. Their systems around the specifications GM DEXRON and FORD MERCON were forerunners and are still adapted by OEMs today. Meanwhile, they have often been replaced by individual specifications from OEMs. Individual performance classes for ATFs are also published by Mercedes-Benz, VW and many Asian manufacturers. The API classes, on the other hand, do not apply to automatic transmission fluids.