This summer a company well known for GA engine mods actually completed FAA approval for a 'drop in' unleaded avgas replacement: https://www.aopa.org/news-and-m

This summer a company well known for GA engine mods actually completed FAA approval for a 'drop in' unleaded avgas replacement: https://www.aopa.org/news-and-media/all-news/2021/july/27/gami-receives-unleaded-avgas-stc https://www.aopa.org/news-and-media/all-news/2021/july/27/ga... It's limited to a subset of the GA engine fleet initially, but the plan is to broaden over the next year or so.