I have a 1997 Seadoo Challenger. I've run the Mercury brand (Quicksilver) for 5 or 6 years now. I use the full synthetic, but you can use the synthetic blend. Synth blend is what is called for by the shop manual. I use the Quicksilver brand because it's the only thing available in my neck of the woods. I get it by the gallon at my local Walmart.
Your requirements for the oil are to ensure it's a TC type oil. You can't use TCW3 or anything that is NMMA certified. Make sure your read the back. On the Quicksilver, it states it fits all requirements for the Seadoo's... It's about $28 bucks a gallon.