Transmission and rear axles transfer engine energy to the drive wheels. Therefore, they are under heavy load. As engine power increases, the load on transmission oils increases. In addition, to improve fuel economy, efforts are being made to minimize power loss in the transmission. Thus, the trend is towards low viscosity oils.
When choosing oil for transmissions and rear axles, the most important selection criterion is the requirement of the vehicle manufacturer. Synthetic oils improve fuel economy, especially during the cold season.
Different types of automatic transmissions (for example, CVT) require different types of oil. The demands of car manufacturers are increasing, so the oil has to be chosen carefully.