There are engines run regularly on 15% nitro 15-20 years old and still going strong. I don't see any negative effect. Oil content is important though. Go with a minimum of 18% of some type of oil content for lubrication and don't run lean. If you have a high compression engine DESIGNED to run FAI (Federation Aeronautique Internationale) fuel it MIGHT effect the engine. They can have compression lowered by adding a shim gasket under the head.