A differential (or "diff" for short) is a mechanical device that allows the wheels on an axle to rotate at different speeds. It's essential for maintaining traction and stability during turns.

A differential should be serviced according to the manufacturer's recommended maintenance schedule, typically every 30,000 to 60,000 miles. Regular maintenance involves draining the old fluid, inspecting the unit for damage or wear, and replacing the fluid with the type and amount specified by the manufacturer.