I think it's more that most aviation piston engines were originally approved for use with leaded fuel, and things have been slow to change. At least some of the

I think it's more that most aviation piston engines were originally approved for use with leaded fuel, and things have been slow to change. At least some of them apparently run fine on the right unleaded fuel. https://en.wikipedia.org/wiki/Avgas#UL94_(formerly_94UL) https://en.wikipedia.org/wiki/Avgas#UL94_(formerly_94UL)