The gears and bearings in the car's differential are worn away by constant friction and heat. Gear oil in the differential lubricates these internal components and gives them a longer serviceable life span. The only difficulty is that unlike engine oil which has a monitoring system that easily alerts drivers if it's too low or too dirty, there really isn't any system that monitors differential gear oil. The driver/car owner will have to directly check the component to see its condition.
- Locate the differential. In a rear-wheel drive vehicle, the differential is positioned in the middle of the two rear wheels. In some models there will be two access points, one for draining the old gear oil and another for filling up the differential with new oil. These holes are secured by either plugs or bolts which can be loosened or tightened with the proper size wrench.
- Check for oil leaks from the differential. You can place a light colored cardboard under the car exactly under the differential and turn the engine on. After a few minutes you can check for any oil stains on the cardboard which would signify that differential gear oil is leaking. Alternatively you can unscrew the bolt or plug in the fill access hole and literally check with your fingers if the gear oil is still up to that level.
- Check for metal shavings and other metallic debris. Some drain access holes on a differential have magnets that attract metal shavings and other metallic debris. These metallic particles are residue from the ground out surfaces of the gear components in the differential. If you observe that a large amount of metallic shavings and debris has collected on the magnet then you can safely assume that the gear oil is no longer providing the necessary lubrication for the differential.
- Change the differential's old gear oil with new oil. Before you execute any oil change, make sure you're using gear oil of the right density which is usually "90-weight." Regular motor oil is only for the engine and will not be dense enough. The other important precaution is to use jack stands on level ground if you need to lift the car to get to the differential. The first step in an oil change is always to drain away all the old oil. Place a pan that can contain more than a quart of oil under the differential before you unscrew the drain access hole. This step could take some time as you'll need to have all the old oil out. When it's done you can close the drain and this time open the fill access hole and pour in the new gear oil there. The amount of new gear oil you'll need to pour into the differential should be enough to reach the brim or just about dribble out.
Most of the wear and tear that happens on the differential's gears and bearings happen during the breaking-in period. When a new car has traveled an average of 3,000 miles or so, this would be an opportune time to check and most probably change the differential gear oil.