Aircraft, or more correctly, aircraft engines that have a STC (supplemental type certificate) to use auto fuel are fairly common. I had a 1959 Piper Tripacer, tail # 2960Z, with a Lycoming 0320 160 hp engine and had a STC to use auto fuel. I switched back and forth between the two fuels with no problem and basically no difference in performance worth talking about. I sold the plane some years ago and I don't know if I would want to use or even be allowed to use the auto gas with ethanol. As others have mentioned, AV gas has lead, which will contaminate catalytic converters. AV gas labeled 100 LL meaning "low lead" still has enough lead to get lead deposits on the aircraft sparkplugs. AV gas, unlike modern pump gas, doesn't go bad as quickly. Race gas, last weekend at Sonoma Raceway in Sonoma California, was $9.99 a gallon for 100, 110, 112 octane.