Transmission oil performs the following functions:
- lubricates parts (bearings, seals, synchronizer clutches, etc.p.);
- creates a protective film on the working surfaces of gears, reducing friction and preventing the formation of scuffs;
- removes wear products from rubbing surfaces;
- cools the parts.
Transmission oil is classified according to viscosity and performance properties.
The SAE (Society of Automotive Engineers, USA) classification determines the properties of transmission oil by viscosity. This standard is internationally recognized and is used by all oil producers. Transmission oils are divided into winter, summer and all-season. The viscosity of winter oil is indicated by a number with the letter W (from English, winter - "winter"). The viscosity of winter oils is determined for four classes - 70W, 75W. 80W and 85W Class 75W allows a temperature drop to - 40°C.
The viscosity of summer oil is marked only with numbers without a letter. Classes 80, 85, 90, 140 and 250 are defined. The numbers characterize the viscosity of the oil at a temperature of 100°C. The higher the numbers, the higher the viscosity.
In everyday use, it is more convenient to use all-season grades of transmission oil. The viscosity class of all-season oil is marked with an alphanumeric index consisting of two values indicating the viscosity classes of winter and summer oil, for example, 75W-80.
The API (American Petroleum Institute, USA) classification divides transmission oils into groups depending on their performance indicators. This standard is also recognized by the international automotive community. Currently, there are six groups of transmission oil, four of which are (GL-1, GL-2, GL-3 and GL-6) are not used in cars. Group GL-4 is used in transmission units with cylindrical gear. Group GL-5 is used in transmission units with hypoid gear. Oil manufacturers often list in detail in catalogs or on labels the characteristics of units and assemblies where the use of each type of transmission oil is permitted. It should not be assumed that GL-5 oil is better than GL-4, since it is designed for higher loads. Moreover, the use of GL-5 oil is contraindicated in some types of gearboxes, where it disrupts the operation of synchronizers.
Transmission oils are produced that meet the requirements of both the fourth and fifth groups. The packaging of such oil is marked GL-4/5.
The specified classifications define general requirements for the performance properties of transmission oil. When selecting oil, car manufacturers conduct comprehensive tests, as a result of which they select the most suitable oils for use in their car units. The vehicle's manual transmission uses a special transmission oil designed to last the entire service life of the vehicle without periodic replacement.
Reference data for servicing a manual transmission
| Filling volume | 1.8 l |
| Recommended SAE transmission oil viscosity | 75W-85 |
| API transmission oil class | GL-4 |
| Tightening torque of the inspection hole plug: | |
| Stage I | 6 Nm |
| Stage II | turn at an angle from 45° to 180° |
| Tightening torque of the breather (filler plugs) | 35 Nm |