20/10/2025
We've got a brand-new "That's Not a Stupid Question" for you thanks to the folks at AMSOIL INC.
Q: Is the fluid in a differential good for life, or does it need to be changed and why?
A: When it comes to your differential, it’s out of sight and out of mind, because it’s under the vehicle. People don’t know whether they need to change the diff fluid, because most people are likely under the assumption that it’s filled for life.
But, according to the experts at AMSOIL, the reality is you can have contamination issues in your differential, so changing this fluid does matter.
All these differentials have breathers on them, and those breathers don’t have filters on them – only a hose that vents, so it doesn’t create pressure and blow out the seals of the axle. This means you can get moisture and dirt in your differential, which causes havoc with oil, bearings and gears.
Dirt obviously causes abrasive wear, while moisture causes viscosity problems. Those are two things you’ve got to worry about from a contamination perspective. As oil ages, it can oxidize and get thicker, and viscosity is the number one important need if you want to protect anything, whether it’s bearings or gears.
Everything in that differential is metal, and therefore, it wears over time, generating small, minute, micron-sized particles of iron. Typically, there’s a plug or a magnet that sits in the bottom of the differential that the OEM puts there to suck out metal in the fluid. Once those magnets get coated up, their effectiveness goes down, and the iron particles are hugely abrasive, so wear creates more wear.
Because of these things inevitably occurring within your differential, changing the fluid out to remove what’s stuck on that magnet and the small particles floating around in solution is a good thing to do.
A differential has a pumpkin in the back, which holds a bunch of fluid that lubricates the ring and pinion, but you also have fluid in the 3-ft. running to each wheel end. It’s not enough to just pull the diff cover off. You’ve got to jack up one side to let the excess oil flow towards the center, drain it out and repeat the process on the other side. Then, when you put the cover back on and fill it, you’ve got to make sure you are getting the right volume back out to each wheel.
Just like when you drain the diff fluid, you should jack up each side of your vehicle to get fluid volume to the wheel ends. That process ensures you’re good to go because it’ll find its own equilibrium once you’re driving around.
Your differential fluid doesn’t last forever. Normal differential fluid service intervals for vehicles is 100,000 miles. However, if your car or truck sees regular heavy towing, heavy loads or lots of throttle, your service interval goes down to 50,000 miles.
This 50,000-100,000-mile interval also applies to those who may not use their vehicle on a super regular basis.
Remember, a diff fluid change is not as daunting of a task as many people think it is. In most cases, the job only takes 20 minutes.
