I have a 92 SR5 V6 with the 5speed. I've been using the same Amsoil now for over 100K. The truck hardly goes out w/o a trailer most of the time. Still have the first clutch with 150K and I replaced the Amsoil with Amsoil again a year or so ago. I never had any problems. My detroit locker actually works better. I can always tell when the locker starts acting up that it's time to change the rear end. Also Amsoil. The older gear box might not be as sophisticated then the newer trucks but I thought they also have the R 150 tranny. Maybe with upgrades.

I have read opinions about the Amsoil and Redline gear oils, but what about Royal Purple? I have been told that Royal Purple motor and gear oils are the best on the market. Does anyone know if there would be the same 'gear slippage' problems? Thanks

No experience with Royal Purple, but they have a good reputation. The most important aspect in the tranny is GL4 vs. GL5. Gear oil for the diff has a very different - and mutually exclusive - function than gear oil for the transmission. Everything you want for the diff works against you for the transmission. That's a bit of an oversimplification, but you get my drift. You want the synchros to shear a bit to grab the gear cones, while you DON'T want any such thing with the ring and pinion.

I think the cause you're buddy is having is due to the reduction in friction between key components. If things are not turning at the proper rate when he shifts, it feels like he's grinding. so the manual gear box is "engineered" to have X amount of friction. so I guess the Amsoil is to good. I've been running Mobil 1 75-90 synthetic in my 94's 5spd and Tcase and axles. no problems. But I plan on going to Redline in all my drivetrain. so i'd go to the Redline to fix your problem. specifically made for gear boxes. b/c friction reduction is good... but not in some cases. case and point your buddy's situation.