One of my least favorite maintenance practices is having to replace the gear oil in a vehicle’s axles. It’s often smelly, messy, and an overall pain in the butt. So when it came time to replace the 80W-90 oil in our 1991 Mitsubishi Pajero‘s axles, I wasn’t exactly looking forward to it. But I had been given a few tips from other gearheads about how to make the process go a bit smoother. I’m happy to report these tips worked and the process was much easier than any gear oil change I’ve done in the past. Hopefully some of these tips will help you next time you need to change these fluids.
Here are a few things you’ll need:
- Wrenches to remove your drain plug and fill plug (mine were 15/16″)
- Gear oil in the correct viscosity and amount (our Pajero uses a little less than four quarts of 80W-90)
- Oil catch pan or something to contain the drained oil
- Disposable gloves (trust me on this one—used gear oil stinks and you don’t want it on you)
- Rags to wipe excess oil
- Gear oil pump
- Ramps / jack / jackstands (optional depending on vehicle height)
Step 1: Before you swap out the fluids, take the vehicle for a drive. This will get that old gear oil warmed up and make the fluid thinner and easier to drain.
Step 2: All differentials will have at least two holes: one for filling the diff, one for draining it, and there is an order to which to remove first. Always remove the fill hole first. This ensures you’ll be able to fill the diff after you’ve drained it. After all, it’d be bad to drain the fluid only to to figure out you can’t get the fill plug out. Opening up that fill hole first will also allow the fluid to drain quicker.
Step 3: Once you crack that drain plug open, get ready for a potentially smelly and messy affair. Old gear oil, especially with a friction modifier, can simply reek. If you get it on pretty much anything, be prepared for that item to stink for pretty much the rest of its life, so try to go slowly. You can collect the draining oil in pretty much anything. I use either an oil drain pan (above), an empty plastic milk jug, or an empty one gallon container of oil with a funnel. All seem to work equally well. If you do use a milk jug or old oil container, be sure not to mistakenly knock it over because it’ll create a hell of a mess. Ask me how I know.
Step 4: Let the oil drain out completely. This might take some time, but the more oil you can get out the better. Once drained, I reinsert the drain plug and torque it down to spec (about one grunt).
Step 5: Warm your gear oil up by placing the bottles in a bucket of hot water. Warming them up will lower the oil’s viscosity making it easier to flow into the differentials. I can’t emphasize how much easier this stuff is to work with when it’s warmed up, especially if it’s cold outside. I’ve pumped gear oil in 40° F (4.4° C) temps and it’s a pain in the ass when the oil isn’t warmed up. Warm the fluid, and it’ll easily flow into your axles. Less mess, less fuss.
Step 6: I generally buy my oil in the one-quart squeeze bottles. You cut the nozzle of the top and squeeze in as much as you can. But you never get all the oil out. To aid in extracting as much oil as humanly possible, buy an oil pump. For about $10 you can simply open your squeeze bottle and thread on one of these hand pumps. You’ll be able to get a lot more oil out of the bottle this way. Once you’ve gotten as much oil out with the pump as you can, open your next squeeze bottle, then remove the pump from the previous bottle and pour in the remainder into the new bottle. Much less fluid is wasted.
Keep adding oil until it starts running out the fill hole. There really isn’t a better way to tell whether your diff is full or not. Once it starts puking out, screw the fill plug back in. Have a rag handy to wipe the excess oil off. Torque the bolt to the correct spec, and you’re done. If you’ve got a 4WD rig, then move to the next differential. It’s that easy.
Doing this yourself can save lots of money on labor. And if you work slowly and carefully, you can keep it from being a majorly messy process. And while I still don’t exactly love changing out gear oil, I’ve gotten much better at it and these tips work.