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I also did not know this, so went digging. Looks like it's just small prop aircraft that still use leaded gas, even though most of them would run with unleaded just fine. How crazy!

Relevant Article: https://www.scientificamerican.com/article/lead-in-aviation-...

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Only newer engines can run unleaded. Most AV motors are made in the legacy style and have unhardened valve seats, requiring TEL in AVgas. Overhaul to mill the heads to install hardened seats would run 8k to over 100k depending on the craft. FAA only regulates when accidents happen it seems, so perhaps reframe the problem. FAA regs requiring proven engines and disincentivising things like fuel injection, electronic ignition, and other staples of ICE efficiency and reliability. We are effectively stuck in the 1950s in small aircraft ownership as a direct result. Considering the advancements, its a bit insane not to at least allow new tech, much less disincentivise it.

Another alternative would be coming up with some organometallic that replicates the properties of TEL, the transition metal(s) of which are something that's not lead. Or mercury, uranium, cadmium, and a few others that are really easy to avoid. Materials chemistry has gotten lots better over the past hundred years, the meaningful development costs are only in validation testing. The FAA "should" be able to cover that cost instead of individual pilots converting their engines.

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