I’m sure lots of companies can make a good synthetic, or synthetic blends. The best fuels for R/C pylon racing are a blend of syn and castor, just in case you get too lean. Castor works great too, but it foams and does tend to varnish more which can reduce engine life unless you like to tear down and clean your engine often (which I’m sure some people do).
Can the folks who had problems confirm that this was 3+years ago as Blink states. If so then this could very well have been a one time problem.
Again, I’d really like to know what Synthetic or Syn Blend I can run and stay in spec. It sounds like RS7 is the current spec, but if the Rotax owners need to run Motul to maintain their warranties then I think we should take that into consideration.