After I moved to the airpark, I started burning about 50/50 av/mogas. One tank was for auto, the other for av. Takeoff/landing was done on the avgas (better vapor lock protection). The engine (O-360, 8.5:1) loved the car gas and ran cleaner. Plugs stayed cleaner, just all around happier and cheaper.
I just paid for the Petersen STC for my Skywagon...$345 for that! (Certified airplanes are a pain sometimes...)
My buddies across the street...Cherokee 235 and a C-120 both have burned mogas exclusively for many years. It's great...but stinky.
Once you go over 9:1, octane becomes an issue. Certified or not, once it's put into the experimental airframe, it is no longer a "Lycoming". It's just an engine. Feed it whatever it likes.