[kedikedi]

I think you’re right. Leaded additive is an octane enhancer to prevent knocking but it’s also good for valve seats and whatnot. That’s the bit what makes it difficult to just switch to other octane enhancers as far as I know.

[denton-scratch]

Suggested explanation: jet fuel doesn't need added tetraethyl lead. But aviation fuel is standardized; it's convenient to use the same fuel for jet planes and planes with piston-engines, so that airstrips don't have to store more than one kind of fuel.

[tass]

Jet fuel and avgas are kept carefully separate and are incompatible - Jet fuel has no added lead.

Most piston aircraft use avgas (usually leaded) though some use jet fuel (diesel powered). Larger propellers are usually powered by turbines burning jet fuel.

[denton-scratch]

Thanks for clarifying!