Rear differential fluid lubricates gears in the differential that allow the rear wheels to turn at different speeds while cornering. In all-wheel-drive vehicles, differential fluid is used in the transfer case, which distributes power among all wheels as needed.
Rear differential fluid naturally moves around the differential as a vehicle is in motion. It lubricates the gears in the differential to prevent metal-on-metal contact that causes premature rear-end wear.
Most rear differential fluids are conventional oils, meaning that they come from refined crude oil. Synthetic gear lubricants are also available, and they offer longer drain intervals and better frictional abilities to increase fuel economy.
Mechanics also call rear differential fluid by other names, including gear oil, gear lubricant and differential oil. Gear oil is similar in viscosity to motor oil, but it is rated on a different system, so typical weights include numbers as high as SAE 90 (compared to such viscosities as 10W30 for motor oil).
You will find a variety of differential fluids on the market. Be sure to use the one recommended by your manufacturer. As oil-manufacturer AMSOIL states, the gear design dictates the type of differential fluid that must be used.
Some manufacturers state that their differentials are filled for life. The only time that a rear differential is filled for life is it is lacks a drain or check plug. Be sure to service the differential fluid as recommended by your mechanic to avoid expensive rear-end repairs.
