There's no problem with my differential. Maybe a little whine when I let off the throttle and coast, but I'm not sure if its the differential, or if its even a problem. The truck has done it for since I've owned it, and hasn't given me a problem or gotten louder. I'm not concerned about that. I just want to make sure I'm keeping up on maintenance, and getting the most life out of my truck. But with 175,000 miles, and having (I presume) never had the diff fluid changed, is there a point? There's probably lots of little metal filings in there just judging by the age and mileage, but would changing the fluid just accelerate wear at this point?

I'm just hoping I won't be making the classic "change the transmission fluid and now it slips" style of mistake.

 

I think fresh fluid in a diff is never a bad thing. The only "negative" I have ever had from doing it is the pinion seal sometimes leaks after getting new lube.

 

Use 75w90 dino oil, it is less likely to leak than synthetic.

 

I agree. If there is a way for a leak, synthetic will find it.

 

Never a bad thing to do all fluids. But I agree, these old trucks don't need or want Synthetic. MAYBE semi syn. I'd still do Dino on the T case, and diffs. BTW the FIRST thing I always do when I get a used vehicle is ALL fluids.

 

I do as well. I didnt on my most recent purchase, and within a few weeks, I was hounding salvage yards for a new rear axle. Seems the p.o. hadnt changed or checked it in a long time, and it ran dry when I was on the way to catch a flight. Figures vehicles fail when you need them the most.

 

They sure seem to! Actually the first thing I do is an oil change, then top off everything else. A few weeks later I have researched the best fluids to replace the stock stuff with, and sometimes iit is just new fresh stock stuff, and then I do a full flush and fill. On the 1991 W150 I have done everything but the rear diff. I did check it when I got it and the fluid was clean and smelled fresh so I topped it off and have yet to replace it. I did do the T case, front diff, and trans as w4ell as all motor and underhood fluids.