The Univ of North Dakota has reverted to 100LL after finding valve seat recession in engines using Swift UL94. The planes were mostly Piper Archers and Seminoles.
This could be trouble. Or, maybe not.
Link to AVWEB piece: Citing Valve Damage, UND Drops Unleaded Fuel And Returns To 100LL (Updated) - AVweb
This could be trouble. Or, maybe not.
Link to AVWEB piece: Citing Valve Damage, UND Drops Unleaded Fuel And Returns To 100LL (Updated) - AVweb