Teague used to send Amsoil out with his upgraded drives, don't know if he still does or not. I have used it in my outboard lowers and have seen a decrease in temperature and shavings on the magnet over the Merc Hi-perf gearlube. I have used the 80W-90 in the past. Just recieved some of the new 75W/80W-90 Marine Gear Lube. It is supposed to work even better in our application, it will still work when subjected to 10% water contamination per Amsoils catalog. It's worth a try.
Jim at www.nwoffshore.com is a dealer and has researched this subject, you might want to contact him.
Just my $.02