In the small motors, most of us use we want the spark plug to ignite the air-fuel mixture at the proper time. If the compression heats the air-fuel mixture to a "flash" point and the mixture ignites before the spark plug sparks, the motor knocks. Fuel w/ a higher octane rating will ignite at a higher temp. allowing the spark of the spark plug to do the job r.ather than the heat of the compression. The higher the compression the higher the heat.I am not a petrochemical engineer, nor do I repair outboard motors for a living. But my understanding (and firm belief) is that there is no benefit to running higher octane in engines of this type (with comparatively low compression ratio). I do have a friend who has owned a gas station for decades. He was the first to tell me this because I used to run 93 octane in everything.
I have no firm understanding or belief as far as downsides, other than cost.
I would absolutely run high octane if I had to in order to NOT run ethanol. In any small engine. Downsides or no.
So Gene's friend is correct. You do not need a high octane fuel in a low compression motor. But high octane (within reason) used in a low compression engine has few drawbacks. The burn temp. of high octane fuel once ignited are within the boundaries of our motors and added deposits in are minimal.
I agree with Gene. The potential of ethanol to cause problems is well documented. I want gasoline without ethanol it in.