Amsoil claims that their oil is compatible (mixes ok) with about everything on the market, but never a bad idea to purge the old oil first. As Lou said, make sure you get amsoil interceptor (golden brown color) and not amsoil injector oil (blue color).
it is a pretty color of brown isn't it ?
FYI Quicksilver is not WalMart oil, it is sold a WalMart among other places, including Mercury dealers, the oil is marketed by Mercury Marine. I've been using it for three seasons with no problems. A lot of other guys have been using Quicksilver. Last I checked the SeaDoo dealer here was selling XPS for $78.50 a gallon.
Lou
Dude you get what you pay for... Wal-mart is one grade above gas station oil..LOL