I have a ".60 size" Great Planes Xtra-300 with a new OS .91FX in it, and it flies terrific ! BUT, I've had so much advice on fuels to use that I am thoroughly confused. The engine has about 3 hours on it and starts fine. My concern is the long term life of this engine. I started using 15% Morgan, but I'm now down to 5% due to very high compression vibration..? I couldn't keep the cowl screws on it ! I'm contemplating buying an OS 1.60 and I'll have the same quandry again. Any advice appreciated, thanks

That's a busy post. Going from a higher to lower nitro% is OK in my book for various reasons that we can get into later if you like.
Personally I don't own or run anything but 5% in my MVVS .40, Rossi .45, OS 1.60 or Moki 1.35. That part is fine.
That being said, I don't think it's the cause of your vibration. All one cylinder engines exhibit a resonant vibration at certain RPM levels. Although we can't change that fact, it doesn't make our lives or cowls any easier. The things we can change or check are things like prop balance, engine mount to firewall and engine mount to crankcase screws for tightness, and crank runout.
Leaning the engine out a little sometimes helps. If any of this makes sense let me know. Take care_Bob

The fuel does not cause vibrations, like Bob said, an unballanced prop or something loose will allow things to vibrate more than they otherwise would.
I know that the various isolation mounts aviable do work well, I especially like the DuBro one.
5% runs great in those engines.

Fuelman