Unless you are independently wealthy, you’re going to be flying a piston plane, which means either gasoline (of some sort) or diesel.

There *are* a few diesel piston engines, which are normally run on jet fuel (aka kerosene), but they’re somewhat rare, and rare means expensive both to buy and to maintain. This is disappointing since these would seem to be the clear path forward, but it’s a classic chicken-and-egg problem.

However, 99% of light aircraft use some sort of gasoline.

Leaded avgas is by far the most common, and also by far the most harmful to the environment. The good news is that most engines under 200hp run just fine on *ethanol-free* mogas; all you need in most cases is an STC and some placards to make it legal. The key is these engines never needed 100LL (or its predecessor 100/130) in the first place; they ran just fine on 80/87.

Note that finding *ethanol-free* mogas can be somewhat challenging, but it is still far cheaper than leaded avgas. And while the very long term future of mogas isn’t great, it’ll be around for the foreseeable future.

For avgas engines over 200hp, which do typically need 100LL, several companies have been working with the FAA on a substitute high-octane unleaded fuel (which is actually not gasoline at all), and there may finally be a light at the end of the tunnel. But as a niche fuel it’ll be expensive, especially if the under-200hp folks all switch to cheap mogas instead. Long term, it would be cheaper to just replace all those engines with diesels.