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.
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 because 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 fluid 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.
When to change differential oil
While an excellent time to change gear fluid 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.
Choosing the right differential oil
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 fluid, 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 fluid 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 (LSD) will sense this loss of energy and redirect torque to the wheel that has the
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
Changing the differential oil
A final note is that all differential fluids possess a particular odor that may or may not appeal to your senses. Sulfur compounds used in extreme
pressure lubricants smell either like rotten eggs or worse depending on additive concentration. A good call is to wear clothes you can throw away and not
get any gear oil anywhere you don’t want to smell it for a while. This applies in particular to the interior of the vehicle. If for some odd reason you
like the smell of rotten eggs, you’ll absolutely love the smell of gear oil. If not, then take the proper precautions to avoid the lingering reminder of a
gear oil change.
Step-by-step guide to changing differential oil
Secure the vehicle on jack stands or ramps. Locate the differential drain bolt. Loosen and remove.
Allow time for the gear oil to completely drain. Replace the drain bolt. Wipe any excess gear oil from the case.
Locate and remove the gear oil fill bolt.
Use a pump to slowly add the recommended type of gear oil to the differential.
Add the recommended amount of gear oil to the differential, or until gear oil starts to dribble out of the fill hole.
Reinstall and tighten the fill hole bolt. Wipe off any excess gear oil from the case.