Quicksilver!
I buy the synthetic blend (which is recommended by Rotax) of the Quicksilver oil. I looked all over the place for the required "low ash," non NMMA certified oil in all the major "boats r us" stores until I came upon the Quicksilver oil in my local Wal-mart store. A lot cheaper than driving 45 miles to my local Sea doo dealership to buy the expensive stuff.
I'll tell you what I read in my shop manual, and this is in reference to the Rotax specifications.
It recommends the synthetic blend, low ash, non NMMA type of oil, but I can use the standard oil blend if synthetic blend is not available. In reading this, it also goes on to explain that if I change to a different type, to completely drain the oil out and refill it with the new type. It doesn't say why, but my guess would be that different oil has different oiling additives than others and that mixing these different oils could change the oils properities, causing the oil to break down and cause engine damage.
Inside your engine compartment at the cap to fill your oil tank, should have a sticker giving the specifics of the main properties of what type of oil to use. I don't really have comprehensive knowledge on the Mercruiser for the sea doo, but I hope this has helped a little.
