Years ago a client of mine was a  chemical engineer at Chevron. He was nearly killed by an exploding test tube while developing the reactants for air bags, but I digress. He loved to talk about his work in developing new ways to make auto fuel (AKA pump gas). As it turns out auto fuel is to a refinery what sausage is to a butcher.  When crude is cracked the easily separated "pure" molecules are drained off first. The "spec" fuels are next, such as lubricants, Jet fuels, diesels, butane, propane and the base for AvGas. What is left  goes through a refinery equivalent of a meat grinder (reformer) until it can be put in auto fuel or heating / bunker oil.( I have absurdly over simplified the process for brevity.) According to this guy, auto fuel is better defined by what is not, or can't, be put into it at a given percentage concentration. Given the variability of the crude feed stock, a refinery needs this hydrocarbon sausage to maintain profitability and dispose of all kinds of  otherwise unmarketable molecules. 

His job, in part, was to develop processes and additives that would allow a broad spectrum of molecules to survive in solution long enough to see Mr. Fuel Pump. Many times these additives would be seized on by the oil company marketing wizards and given whimsical names like "Tecroline" and then make claims that this fantastic additive would prevent injector fouling, improve mileage and promote smother running. All true...BUT the full truth was that the refinery had a couple million barrels of a sticky hydrocarbon that they had to get rid of but when the put it in auto fuel it would varnish out the injectors so they needed the wonder additive or your car would stop running after a couple of tanks full. Bad for business. A dirt sandwich with Tabasco tastes better than a plain dirt sandwich, but you are still eating the dirt.

So, saying you are putting auto fuel in your airplane is a little like saying you are going to San Francisco for sex. No telling what you will get or, in the fullness of time, if you will still think it was a good idea.

Regards
Brent Regan