Auto Systems and Repair: Differential Drain and Refill
The differential contains gears and bearings that are lubricated by oil. Like all oils and fluids in a vehicle, the oil in the differential should be changed every 30,000 miles or more frequently if you use your vehicle to tow or carry heavy loads.
To drain and refill the differential fluid the vehicle is driven to get the differential oil warm. The oil is then drained from the differential and refilled with fresh oil.