Clancy,
I used OS 91 Surpass engines for flying Senior Pattern for some years. I used Morgan 30% Heli fuel (all synthetic lube) with complete satisfaction. The higher nitro gives 300 or so more rpm but the biggest benefit was much better idle and throttle transition.
No experience with 10% nitro in this engine, but 20% should be OK if idle and transition are important to your application.
Cheers,
Jim