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What's the Difference? How to Change Differential Oil
By Mike Bumbeck/
Printable version
Degree of DifficultyEasy
Estimated Time45 minutes
45 minutes
It's time to face facts. Every once in a while your rear end needs a little help. Whether you drive a rear-, front-, or all-wheel-drive vehicle, the wheels spin by way of a differential. The gears inside the differential distribute engine energy to the axles and the axles spin the wheels the right way at the right speed when you hit the gas.
Changing Differential Oil
Another important function of the differential is to allow the drive wheels to spin at different rates as you round a corner. The inside wheel spins slower than the outside wheel as it travels a shorter distance around the bend. From this difference, the differential gets its name. The gears inside the differential slide around coated in a slippery film of oil. Just like engine oil, the differential oil must be changed at regular intervals. Not quite as often, but at regular intervals nonetheless. Without the lubricating properties of this high-pressure gear oil, friction will quickly wear through the special layer of hardened steel on the gear teeth, and the gears can fail prematurely.

Extreme Function
While an excellent time to change gear oil is when the differential has to be drained for axle service and the like, it's always best to consult your owner's or service manual for proper intervals. Not just any oil can stand up to the extreme pressures dished out by the constant lashing of the differential gears. The oil must be able to withstand the shocks and loads created by the transfer of engine horsepower and torque to the wheels. Right up until commercial whaling was largely outlawed in the 1970s, a major component of this high-pressure lubricant came from sperm whales. These compounds have since been replaced with more modern manufactured equivalents.

The Right Stuff
The first consideration is proper weight, or viscosity. The next is the GL rating. Without getting into a lecture on lubrication, the best path to take is to use only what is specified for your differential. If the manual calls for a GL-5 rated 90W gear oil, then that is the only way to go. Second-guessing the engineers that designed the differential and filling up the case with the wrong gear oil can ultimately add up to an expensive guess.

Another important consideration when it comes to differential gear oil is the requirements of the limited slip, or traction-sensing differential. In a normal or "open" differential, the torque, or twist, created by the driveline is always applied to both wheels, regardless of whether one of those wheels happens to be spinning helplessly on an icy surface. A limited slip differential, or LSD, will sense this loss of energy and redirect torque to the wheel that has the most traction.

While there are various types of mechanisms used to accomplish this miracle of redirected traction, most of them require friction modifiers unique to their own design to work correctly. An LSD without these special lubricants will at best not work correctly, and at worst fail outright - ending up costing a bundle to rebuild and repair. If your vehicle has an LSD, always be sure to use gear oil that contains the correct friction modifiers for that particular LSD.

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