I run my A15 on premium 98 octane and it never caused any problems. my carby on the other hand runs really rich so after driving for a bit ( like 2000kms or so ) on long trips it will gum/ fuel up the plugs. this is heaps worse on short drives or lots of idleing. i would check your plugs and at least try clean em or better yet get new ones. as statded before ethanol will do bad things to your car but simple higher octane pump fuel wont hurt.
Avgas is to high octane for a stock motor though.