GEAR BOX & DIFFERENTIAL LUBRICATION
Gears transmit power from the engine to the wheels of the vehicles. At the time of meshing of gear teeth the pressure between them is very high and requires a lubricant film that does not break at such high pressure, the film must be strong which means it must have high viscosity and enhanced with wetting additives commonly referred to as extreme pressure additives. These extreme pressure additives are commonly made from phosphorus and sulphur additives.
The pressure in the gear box is not as high as in the differential. The additives in the gear box are lower than in the differential oil. As we saw, the additives have sulphur in it and sulphur under high temperature and high concentration will react with synchronizers made with yellow metals (copper). It is therefore recommended that gear box oil should be of API Classification GL4 SAE 80W90 or SAE 85W90 or SAE 90.
On the other hand the pressure in the differential is very high especially if you consider that the gear system is hypoid. It therefore needs an even stronger film to prevent the gear teeth welding in each other and tearing out. This film is obtained by using even heavier base oil than in the gear box oil. This oil should have a higher concentration of extreme pressure additive and as all parts in the differential are made of steel, there is no danger of corrosive wear from the sulphur in the EP additives. The oil recommended would be API GL5 with viscosity SAE 85W140 or SAE 140. It is recommended never to use a gear box oil in a differential and vice versa.
In many Chinese vehicles the gear box has steel synchronizers and therefore will not react with EP additives. In this case you can use an AP1 GL5 SAE 80W90 or SAE 85W90 in the gear box and differential.
Our product; Slusol Ciroco 85W90, is designed for Chinese trucks. It is API GL-5.