Break in...
The XP-S formula is actually only required in the first break in tank. After that, if you choose not to run the XP-S formula, you'll have to use an oil that meets Seadoo's requirements.
The oil requrired for the Rotax 2 strokes is a non-NMMA, non-TCW-3, low ash oil. But if your engine has used the XP-S since it was bought and is the original engine, I wouldn't change because the engines wear has become acclaimated to the additives of that specific oil.
Other brands, for those that live to far away from a Seadoo distributer is Amsoil 2-cycle, low ash or QuickSilver. Personally, in my 1997 Rotax, I use the Quicksilver "Full Synthetic" blend. They make two types. The standard synthetic, which has other bases in it and the "Full Synthetic". If you see these oils in Wal Mart, read the back of the label and you'll see where it is useable in Seadoo's.........now, for those that have said this is "taboo". If it weren't a useable product for the Seadoo 2 stroke, don't you think Seadoo would be sueing Honda for putting their name on the back of their bottle?