Dear Tom and Ray:

I have a general maintenance question for you guys. Every time I go to get the oil changed on my 1998 Toyota Tacoma, the service man always tries to sell me an oil change for the differentials. I always decline, because I don't want to pay the money and I'm not sure if the truck needs it.

So, how often should the oil in the rear and front differentials be changed? I've tried to find the answer by looking through manuals but haven't had success. The truck was bought used two years ago, and I don't know the last time the oil was changed for the differentials.

-- Mike

RAY: I'm guessing they got a clearance price on 250 cases of differential oil, Mike, and they're trying to move it out the door so they can fit the soda machine back in.

TOM: You have very standard differentials, Mike. They don't have limited slip or anything fancy in there. They just have a ring gear, a pinion gear and a set of spider gears. I'm sure there are lots of Tacoma owners who never change the oil in their differentials.

RAY: That's what my brother would do if it were his truck. Of course, he takes the same approach to his underwear. But if it were my truck, and I wanted to be diligent about it, I'd probably change it every 60,000 miles or so.

TOM: That doesn't mean you should ignore the differentials for 60,000 miles.

RAY: Right. Because the biggest danger is that they leak and you don't notice it. And then they would run out of oil and fry themselves.

TOM: I did that once in my old Suburban. The differential ran out of oil and started making a sirenlike whine all the time. I eventually went into hiding, because I was convinced the cops were chasing me everywhere I went.

RAY: So it would be very wise to check the oil level in the differentials every time you get your oil changed. That'll allow you to catch any problems early.

TOM: There are other vehicles with clutch packs, viscous couplings and more complicated differentials that do require an oil change every 30,000 miles. But yours is not one of them. So here's what I'd do in your case: Given the age of your truck, and given that you don't know if the differential oil has ever been changed, I'd say you should let your guy change it once. It certainly can't hurt anything.

RAY: And from then on, have him check the oil level for you when you see him. If you still have the truck 60,000 miles from now, and it still casts a shadow, you can decide whether you want to do it again. Good luck, Mike.