The following are quoted from the American EAA's website...
EAA began testing and evaluating alternate fuels for aircraft piston engines in 1964. These included not only automobile gasoline, but also ethanol. In 1982, EAA successfully changed FAA policy to consider the use of automobile gasoline in aircraft. The EAA type certificate program resulted in an FAA Supplemental Type Certificate (STC) being issued for the use of automobile gasoline, rather than aviation gasoline, in the Cessna 150 aircraft, powered by a Continental O-200 engine. The first pilot to legally fly in an aircraft with automobile gasoline was, then FAA Administrator, Lynn Helms. Since then, more than 40,000 STC's have been issued.
Please note that our STC's are only for engines that are certified for the use of 80 octane fuel. Engines requiring 100 octane fuel must still use 100 octane avgas.
There's a great deal more information at
http://www.eaa.org/education/fuel/index.html
However AC-DC highlights a number of the concerns raised by the UK CAA a couple of years ago when we found out that 4* was becoming unavailable. I'd point out a few points...
- Unlike the USA, all of Europe uses a (fairly) rigid standard for unleaded fuel in EN228
- European Unleaded at 92 RON is a higher Octane than the American fuels which are around 87 RON I think.
Of course, this just highlights the fact that it isn't a simple issue. If I ran a Lycoming and flew from licensed airfield, I'd probably use AVGAS. I however run two Rotaxes from a club strip, and unleaded MOGAS is the only readily available fuel. Other people will be in different circumstances, and really need to look into the issues for their own case.
G