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Another issue is that replacements for TEL that were fine for cars had issues with high altitudes (and high ambient temperatures)

It's about detonation; I don't think it's about the valves.

The 100 octane is about detonation; the lead is about lubricant for the valves.

It's about the valves.

Plenty of 100+ octane unleaded fuels out there.

Post-WWI aviation engines overwhelmingly have hardened valves (and valve seats) which work fine with unleaded fuel.

And the spark plugs unhappy?