Aside from the Ethenol issue, there is also the problem of exhaust valve seat errosion, de to the absence of lead in the mogas. I always ran my Luscome on mogas (Continental a65), without any apparent problems. However, I have just taken my Bolkow junior for its annual, and the engineer has found low compression in one of the cylinders, which he feels sure will turn out to be an exhaust valve. The engine is a Continental 0200, with only 175 hours from new. The engineer asked if I had run the engine on Mogas. In truth, I have only done 11 hours in the aircraft, but I know the previous owner did sometimes use mogas, and had even obtained appropriate certification to use it. The engineer feels sure the problem will turn out to bee exhaust valve, valve seat errosion due to the lack of lead in the mogas. This reminds me of the problems some classic car owners had when leaded fuel was phased out. E-type Jaguars were effected particularly badly, as they really did need the old 5-star fuel. The only cure for the E-type was a cylinder head rebuild with new hardened valve seats, lowered compression via a new head gasket, and a change to the ignition timing. All this allowed the engine to run smoothly, but certainly took the edge off the performance of the 3.8 litre engine (the best one!). I will not be using mogas in my 0200 any more.
I also note that someone has commented on organic growths in fuel due to the bio fuel being added during the refining process. During my career as a Transport Manager, operating large fleets of commercial vehicles, I experienced this problem first hand, particularly when we had bunkered fuel in our own tanks. Although I had filters in the tanks to prevent these growths reaching the fuel systems of the vehicles, the growths were so bad that the quickly blocked the filters. In the end i was changing the tank filters every week, and paying thousands to a specialist company to clean out the tanks every three months or so. One of the engineers from the company that cleaned the tanks told me that he had heard of an instance where a company did not have the tank filters and the organic growths reached the fuel systems of their fleet of vehicles. The end result was that two virtually new commercial vehicle diesel engines were written off, at a cost of £15,000 each. Incidentaly, he also advised me that these organic growths seem to thrive in diesel much more than petrol. So much for bio-fuel!