A while back I got a call from a neighbor who had purchased an aircraft rated to burn auto gas.  Our brief conversation led me to add the following blurb to the 'sometimes useful information/opinion' area of the website.

The use of auto gas in 80-octane aircraft is definitely one of those "old wives tales" subjects.  If you mention to more than three or four pilots that you burn auto gas in your aircraft you will be statistically certain to at some point receive a lecture, delivered with religious fervor, telling you that if you let a drop of auto gas get into your plane's tanks you are facing certain ruin, if not instant death. Over the years I have learned that whenever this happens, the best thing to do is just terminate the conversation ASAP.  Why this subject is so hotly contested by some pilots I don't know, but for what it is worth, the following is a recitation of my personal experience with it:

Although I am retired now, for over 15 years I flew to work in my 1965 Cessna 182 on a daily basis.  Summer, winter, day, night, IFR or VFR, I put in five (or more) round trip flights per week.  All of this was done using automotive gas, a total of something over 40,000 gallons of the cheapest regular-grade automotive gasoline I could find (Arco, mostly).  This does not include non-commuting flights during which auto gas was burned, which probably averaged an additional 50 hours per year. I continue to use auto gas to this day whenever possible. (Update...the old C-182 became history a year or so back - see http://jandd.org/requiem.htm - so now I'm stuck with 100LL while it lasts)

During all this flying, I never had a problem related to the auto gas.  The C-182's original O470R engine (1500 hr rated TBO) was still going strong at 2600 hours since overhaul, to which you could probably add its initial 1550 hours, predating a terse logbook entry which merely notes an 'engine overhaul'.  Field overhauls without supporting documentation being what they are, you can be sure it actually had 4150 hours on most if not all of it's moving parts.  During that time its only 'internal' repair was the removal of one cylinder to have a slightly leaky exhaust valve re-seated, a job which cost me a bit over $200 (I did the pull & replace).  However, by 4150 airframe hours and some 30 years of service, the engine was seeping oil from every seam, and in spite of excellent compression readings I couldn't help starting to wonder how long could it possibly last.  In essence, I just got tired of waiting and swapped it out with a zero-time Continental factory overhauled engine.  That factory reman now has passed 1700 hours and except for accessories has never been touched.

The 'anti-auto gas' crowd usually trots out a couple of arguments, prominent among which are the possibility of vapor lock due lower vapor-pressure with auto gas, and the possibility of detonation due to supposed marginal octane rating. 

The C-182 has a carbureted engine which is  fed by a gravity-flow fuel system (no pump), so any vapor lock problems cannot be corrected by applying boost pump pressure.  I have flown the aircraft many, many times out of Bullhead City, AZ, in air temperatures up to 120+ degrees with the controls literally too hot to touch (damp rags work).  The gas, heat-soaked for days on the blazing ramp, probably was at or above 120 degrees itself.  I climb leaned to max power, often to 10,000 or 12,000 feet to find smooth air across the desert. If it was going to vapor lock under any likely conditions, it certainly would have done so during these operations.

Regarding detonation, the Bullhead City operations model addresses that also.  In addition, at cruise I habitually run "over-square" manifold pressure vs. RPM and lean to the edge of engine roughness (carbureted, remember), with no detonation problems.  The initial testing which led to the issuance of the first auto gas STCs for aircraft went much further.

From this recitation, I think you can tell that I feel I have "tested" auto fuel in aircraft service pretty thoroughly.  I can't argue with the "you're gonna die" crowd, because of course they will be proven correct.....eventually.  But I really don't expect it to happen as the result of burning auto gas!

John Wilson & N-8514S