I was thinking about getting some unleaded to a Rotax engine, and realized I could ferry some UL94 in the tip tanks of something like a Bonanza. Not that it really matters in real life, but let's say that I were ramp checked on my way out of the airport and I didn't have an STC for said UL94 (although it could be any fuel at all which is not approved), would I be legal or not?
And if the tip tanks are allowed, then what about filling one main tank on, e.g., a Cherokee and putting the fuel selector on the other?
That's not to say this is a smart thing to do in general (be careful of ethanol!), but if I can transport the fuel in a jerry-can in the back of the plane, it's obv. not forbidden to use the airplane to carry tanks of non-approved fuel. And in the specific case of UL94, it's approved for basically all GA pistons, so we can be sure it won't have any adverse affects on the tank sealant.