Just had an auburn limited slip put in with all new bearings, seals and switched to 3.55 gears on a 8-3/4 489. The gear guy who did the work recommends synthetic, while Auburn recommends conventional with their additive. I am leaning towards Amsoil Severe Gear 80w90 synthetic, but wanted to hear everyone else's opinion. Anyone using synthetic? any noise or other issues? If you went with conventional gear oil, did you research any of the brands out there and which one did you pick?