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.