While all indications (and logic) say use 80w90 GL-5 oil, I confess that at about 10,000 miles ago I used the same oil that I used for the engine - 10W-40 or 15-40 Rotella at the time. Now I use Mobile 1 75W-90 gear oil for the rear gear and Mobile 1 10W-40 for the engine (getting hard to find...