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A recent patent from British Petroleum makes a disturbing admission:
piston-engined aircraft need such high-octane fuel that tetraethyl lead is still
added to it to keep its octane rating up. So while all new cars can now use
unleaded fuel, aircraft engines are still dropping lead from the sky. In WO
98/22556, BP claims a new high-octane fuel which is a mix of triptane,
iso-pentane, iso-octane, toluene and methyl tertiary-butyl ether. This, says BP,
raises the octane number to over 100, which makes it a usable lead-free aircraft
fuel.