> nor necessary for all grades of fuel. That's moving the goalposts. Piston airplane engines typically (but not universally) have high compression ratios, so

> nor necessary for all grades of fuel. That's moving the goalposts. Piston airplane engines typically (but not universally) have high compression ratios, so they require fuel with a high octane rating. In fact, the only avgas that contains lead today is 100LL (100 octane, low lead). If you want to have a discussion about lead in avgas, you're having a discussion specifically about 100 octane fuel. > What about iso-octane? Definitionally, 100 octane fuel has anti-knock properties similar to 100% iso-octane. You may be able to make an approximately-100% iso-octane blend for lab tests, but it's not really possible to manufacture it in commercial quantities. Keep in mind, "premium" auto gas is usually less than 93 octane, and even that has ethanol.