In a differential, gears distribute energy produced by the engine to the axles, which transfer the energy to the wheels. Differential gears must be kept lubricated or they will swell with heat and seize up during operation. Differential oil is designed to operate under high pressure. Differentials in different cars may be designed to operate in different environments. For example, the differentials in a 4-wheel-drive off-road car will not have the same differential oil requirements as a race-track sports car. Differential oil should be changed at regular intervals.
Raise the vehicle on jack stands or ramps. Make sure that the vehicle is stable. Place a container underneath the differential case to catch the oil. Loosen the differential drain bolt using a socket set or wrench to allow the differential oil to drain out.
Secure the drain bolt back in place once the oil is completely drained away. Clean any oil from the differential case.
Find the gear oil fill bolt and remove it using a socket set or wrench.
Fill the differential with the correct amount of oil using a hand pump. Perform this operation slowly and with care until the differential case is full.
Replace and secure the fill hole bolt using a torque wrench. Tighten to manufacturer's recommended limit. Clean any excess oil from the case.
Always use the type of differential oil that the manufacturer recomendeds. This is identified by a rating, such as GL-5 rated 90W Gear Oil.
Correct disposal of used oil is imperative. Improper disposal has an adverse effect on the environment, and in many countries is a criminal offense.