In ground school it's taught that if you are to use fuel different from what the engine normally runs on, use higher performance grade. It's more resistant to detonation and pre-ignition.
Even if the engine can take other than POH approved fuel, maybe some other part of the fuel system can not. I don't have the experience so I can tell...
Where I come from, some operators fly the Cessna 172 (Lycoming IO-320) on 91/96, a lower performance grade than the POH approved 100LL and 100. I have never heard about any STC for 91/96, but it exists for MOGAS. But it works!
Remember, if something happens to your engine during your flight, YOUR ass is on the line since you did not follow the POH.