Some years ago our group used to regularly use MOGAS (the old MOGAS) in our A65 engine. The aircraft used to fly ok, but there were frequent problems with sticking valves etc. When the airfield's MOGAS supply shut the group changed to AVGAS exclusively. After a while, it was noticed that all the previous engine problems had disappeared. Guess which fuel has proved the most economical.
However, I know another owner who has been using MOGAS (unleaded) for over six months now in his C90 engine and (so far) he hasn't had any problems. I've never known him fly above a few thousand feet though.