Rear Differential Fluid

I need to change the Rear Diff. Fluid in my 00' Z71 w/ locking differential. What weight gear oil do I use? Last time I had it changed, the dealer did it, and put in a synthetic oil. This time around I wanted to save a few bucks and do it myself, but the dealership was no help. They didn't know what "weight" or "kind," all they wanted to do was sell me their brand for $27.00 a quart. Any help would be greatly appreciated.

You are probably looking at an 80w-90 weight oil in the rear diff.However if it is a locker you may need to add special friction modafiers to the oil to allow your diff to function properly. Royal purple makes high end diff fluid with the friction modafier allready in it. I'd go with their stuff or the dealers, The dealer should be able to tell you what the weight of the oil is though

They should, and can. They just don't want to because they can make money off you if they change it.

I have an 86 with a "locker", it takes 80w-90, and REQUIRES a friction modifier.

75-90W Mobil One synthetic without additive modifier is the way to go. There is even a TSB out saying that this is best way, reguardless of what your owners manual says.80-90w GL-5 stalube is junk.

75W-90 is the recommended weight. They did not know what weight to recommend because GM uses the same 75W-90 in all of the Silverados. With the GM stuff, you do not need any additive. I have heard of people having success with Mobil 1 and Royal Purple as well. Given that I'll probably only be changing the fluid once in the life of my truck, I'll probably just look for the best price possible on the GM stuff (~22 a quart is the best I've seen).