Engine oils for automobile transmissions conform to the most widely used API standard in the world, developed by the American Petroleum Institute. The abbreviation GL followed by numbering is used to decipher the designation of transmission oils. For gearboxes of old vehicles the categories from GL 1 to GL-3 are applicable. The GL-4 and GL-5 grades are for modern vehicles.
It would seem that the newest and most modern development GL-6 is not used, because GL-5 oils meet the modern requirements to the fullest extent.
The API and SAE at the end of the last century offered universal categories of gear oils PG-1 and PG-2 for general use. The first category is designed for gearboxes of buses and heavy trucks. For drive axles in buses and trucks is category PG-2, which in technical literature may be referred to as GL-7.
The API classification classifies gear oil grades according to their extreme pressure properties. The effectiveness of the additive providing such properties is determined by the GL group number. The higher it is, the more effective the anti-seize properties of the additive, which contains sulfur-based compounds for this purpose. In extreme operating conditions the additive changes the chemical composition of the metal and transforms its surface layer into a modified film, which is a product of wear. This corrodes the metal, but the overall damage to the part is minimized.
Chemical modification is effective only on the surfaces of parts and assemblies made of steel and cast iron. Non-ferrous metals, which are used in the transmission of front-wheel drive VAZ, from such additives wear out very quickly. Therefore, only a conventional engine oil is suitable for them.
GL-5, in spite of its high class, sometimes hinders gear shifting. The reason is the decrease of friction coefficient between synchronizers. Nowadays the most universal are GL-4 greases, which provide optimal synchronization in gearboxes of front- and rear-drive vehicles.
The deciphering of gear oils by SAE classification defines their viscosity. The letter W (e.g. 70W) means the product is designed for low-temperature applications. The absence of W in the marking indicates belonging to the summer grade. Very popular are all-season gear oils marked, for example, 80W-90. Their use relieves the car owner from seasonal gearbox oil changes.