both tanks are sealed (caswell), but i'm always paranoid ... i have not had a problem with caswell in fiberglass tanks yet - i have one that's been sealed for 10+ years without a problem - but there's always a first time. options : pump gas and drain tanks when not in use/leave cap off. fuel will be in tanks a day or 3 maximum. or c12 and drain tanks anyways, i just don't gotta be in such a hurry about it. with c12 i'll probably have to rejet. or avgas? i've never run it.... but supposedly there's a local place here that will let you pump up to 5 gallons into metal container. just walk right up. c12 = expensive - $10 - $12 gallon, gotta drain it anyways. might get some bonus hp. probably not much. avgas = cheaper, but i've not used it before regular old pump gas = cheap, gotta drain it, paranoid about fiberglass.... is there anyone here who regularly uses a proven fuel in their fiberglass tanks?