Oil is muy important! However, besides normal wear (see compression checking), check your tune carefully. Running a 2-stroke lean of fuel for example is most often responsible for killing them. The tiny carb passages corrode imperceptibly over the decades causing a lean condition and the fuel filters load up with junk, the fuel line fittings and fuel tank switch leak air internally, starving the fuel into lean-out causing lean hesitation, imperceptible detonation and overheated (eventually melted) pistons. so that even the best oil cannot survive these conditions.
The Seadoo/Rotax oil injection combined with the correct oil works great as long as it's not neglected. IMO, the problem with ditching the variable ratio is you don't want or need that much oil at low speed and it won't be a good experience.
There's something to be said for the knowledge passed on from old dirt bikers like Gordon Jennings, it's not just about throwing tons of oil at these, far from it, don't fall into that trap. Make sure your tune is correct, lean hesitation is your big warning.
Might be difficult getting them to go slow. 