Just be careful guys. The Merc High Performance stuff is especially designed for use in these drives. It can tolerate significantly higher water loads than the Amsoil. I had a guy running is salt water completely take the lower section of a drive off (exposing gears and bearings), kept using the boat for the rest of the day thinking that as the prop still went round and he couldn't see a problem, then not bring it to us for a week. Using the Merc oil (we'd done the servicing) and not a single spot of rust on any exposed surface. I was shocked, and also a compelte convert to using that oil in drives from that moment forth. Lubrication wise, the Amsoil is probably just as good as the Merc oil, but 'stickiness' to the metal surfaces in salt water exposure, I seriously doubt it.
just my $0.022 (Inc GST)
Chris..........