Parafin, Petrol, probably ground up ducks, a decent 2.25 will run on just about anything, there is of course some measure of how well it will run. The newer fuel types are of course higher octane, and when trying to tune Petro (way back in the day) you come to the conclusion that it's not designed for the later fuel types, since I think it only goes as high as 85 octane, and super unleaded is around 97 :) Still, the better fuel types will likely give you more grunt at little change to your MPG, the only qualm I had was whether the head could take it, since it's not leaded (so there may be some seat damage) and presumably it will also run hotter..
Still, I never had much trouble, and the 2.25 is practically indestructable, so you should be fine.
how much of the "difference" is a placebo effect we may never know, but even if it is, if it keeps you happy who are you to argue ?