Changed the diff. oil in my '85 GMC's 14-bolt axle this week. Coming out: Super S conventional 85w140 with almost exactly 30,000 miles and 7-8 years? on it. Amsoil SG 75w110 went in. I changed the engine oil at the same time - Schaeffer's 7000 10w30 with 14 months? and 6,400 miles on it, with the big Napa 1794 filter (total fill was 7 qts.!) in and out, and afterward I swear I've gained over 15 horsepower. It requires noticeably less throttle, feels as good or better at 60 as it did before at 55, and actually coasts to some extent when I take my foot off the gas. I glanced at the gear wear pattern while I had the rear wnd open - coast pattern looked very well centered but the drive side was toward the outside and a couple people on the Chevy truck board have said it looks like something was causing scuffing on the gear teeth which couple explain the extra drag, too. Pinion bearings and gear slack are all good and tight, no grinding noise (just a little gear hum at low speeds), and no excessive amounts of metal in the old fluid, but the whole inside of the diff. had a black pasty coating like I'd expect from a well-used LSD. Truck has 125,000 mi. on it, and I assume the only service to the axle is what I've done which is (now) two fluid changes and replacing the pinion seal after a period of massive leakage. What do the oil gurus have to say? I'd heard that Super S was decent lube, but it looks like that may not have been the case.
