Wholehog, the con-rod is weak. I found that out when my son was trying out props on my Super-Vee 27 Nitro. It'll swing a lot more prop than it should but the rod isn't strong enough.
Exactly, don't let a hobby shop try to unload just any fuel on you just to make a sale. Most are clueless when it comes to boats. Heli fuel is the next best thing if you can't get a boat blend.
There is also a myth that if you run a higher percentage of nitro that you'll blow up an engine faster. No, in most ways. Higher nitro requires a richer fuel mixture so that in turn adds more oil. It doesmake the needle a little harder to set though and if you live in a hot and humid place it can make it harder to get a good setting.