In general, air cooled engines need higher octane gas because of higher cylinder heat temperatures. Follow the STC. There was plenty of testing done, and engines, airframes and octane limits were the results of the testing. Trying something different will probably not put you into unproven territory. It will probably put you into disproven territory.

Ethanol is bad news. It does attack older rubber. It also causes the engine to run leaner. It also reduces power output. And the worst problem is that it holds water. How much water is temperature dependant. A saturated mix will shed water as the fuel cools as you climb, potentially flooding the carburetor with water and causing an engine failure.

Octane requirements and ethanol prohibition were far from flippant decisions.