When turning a corner, the outer wheels of your car travel further than the inner wheels. The differential gear set allows your car’s wheels to rotate at different speeds so the vehicle continues to ride smoothly. Differential fluid or gear oil protects those gears.
Eventually, the protective additives in gear oil break down. Replacing the gear oil at the manufacturer’s required service interval will help to prolong the life of the gears and bearings within the differential ensuring your car’s turning remains nice and smooth.
If you are using your car for towing or off road use, increased service intervals are strongly recommended to at least once a year.