Just my thoughts.  I had the STC from Petersen on a 1974 C-150, and as I recall there was no actual change to the airplane.  Just some paperwork.  However, most auto fuel available today has alcohol mixed in it, which is not approved for use even with the STC.  I understand that alcohol is a problem in aircraft fuel systems primarily because it can hide water in the fuel, and is rumored to attack the rubber hoses and seals.

I think if it was my airplane, I would drain the fuel just so I wouldn't have the liability (FAA and Insurance) should something happen.  Of course the other problem is that you can only use the mixed fuel in an engine that doesn't run a catalytic converter for pollution controls, which pretty much limits it to a lawn mower or portable generator (in California anyway).

Jon