I've generally gotten the fuel system under control, but I can't find out what aviation fuel does; instead of unlocking anything, it just says "Content unlocked!" and doesn't do anything. Am I supposed to unlock something later on that I can use aviation fuel for, or is it a WIP?

Yes, it is a WIP, just dump the fuel you don't use away, there's no cost for polluting the environment. Only useful fuel right now are Gasoline and Diesel