Which gear oil?

[Skunksmash]

I want to change the differential fluid in my bone stock 87 R-10 pickup. I like the royal purple idea because they say that no additional additives are necessary, and I've heard people say its decent stuff. But which weight do I use? I'm thinking just the typical 75w90. Royal purple is what I've always used in these old trucks, just wondering if its actually the correct weight.

[VileZambonie]

GM recommended 80w90 GL-5 in the rear and 75w90 GL-4 in the front in cold climates otherwise GL-5. As far as additives, if it's the G80 locking differential no additive is recommended.

[Skunksmash]

Thank you sir. Much appreciated.

[Skunksmash]

Ugh. I'm having trouble finding that in a full synthetic. Would it hurt anything to use 75w90 instead?

[VileZambonie]

Your hypoid gearset and bearings could care less. 75-90 80-90, 75-140 synthetic, conventional it doesn't matter as long as it's an EP hypoid Lube you're fine. Why are you heck bent on synthetic?

[Skunksmash]

Had a couple of other vehicles give me good results. Neither one were one of these old trucks, but I figured I'd try it none the less. Now maybe the old fluid in those other vehicles was just finally wore out, who knows. But it was a noticeable change so I'm going to take any good luck I get and run with it.

[jaredts]

A noticable change in what?

[Skunksmash]

Noise from the rear end. My 94 GT mustang was slightly audible. Once I put the synthetic in, the sound was gone. That noise is what led me to change the fluid. The car also seems to get RPMs slightly faster now, and will burn out easier as well. In granny's lincoln, it actually affected the gas mileage. Made it get about .4 better. These days, that's money in the bank.

[zieg85]

When I change my ring and pinion over I am going synthetic in the differential.  Nowadays it isn't much more $ to go that route.

[Skunksmash]

Yeah I'll take the benefits for a few bucks more thanks. Some things ain't worth it, but some things are.