It is a common misconception that higher octane number means higher performance. Higher octane burns cooler and more slowly to prevent detonation (knocking) in high compression engines. In lower compression engines carbon can build up because the combustion is incomplete. I would suspect ring fouling and other problems could occur.

An interesting aside to this topic. I was changing the plugs, and found they were fouled, so they would not back out. The threads go past the head in the combustion chamber, so they get carbon on the treads. This is evidenced by being able to easily back out the plugs a few turns, then the gall up. If this happens, tighten them back up, and get some RTX (I think this is the name). It is a very small bottle, in the chemical isle, that will remove carbon. I am told that Chevrolet makes something like that also. Ask at the autoparts counter, they can help you find it. The bottle is very small and has red, yellow and black on it. With a full tank, put about a table spoon in the gas. Don't get any on the paint. Run that tank of gas out, and try to get the plugs out then. It should be easy now. DO NOT FORCE THE PLUGS OUT.