A little background on Avgas:
Avgas cannot be oxygenated by specification. There is an option to use either isopropyl alcohol or DiEGME [diethylene glycol monomethyl ether] as icing inhibitors, but they may only be added to a maximum of 0.15 vol% - certainly nowhere near the automotive oxgenate concentrations of 10 vol% ethanol or 15 vol% MTBE. Avgas is used in applications where the engines are air cooled, cylinder head temps in the range of 475F, and power settings of 100% for takeoff and climb, 75% cruise, and 55%-65% for "lean" cruise. The engines last 2000 hours between overhauls and live most of their lives at these settings. The biggest difference between avgas and autogas is composition, and thus boiling range. Autogas can have an end point of 437F (often, it is around 400F), but avgas must finish boiling at 338F or less. This requires avgas to contain only lighter materials, but because of the high octane requirement in the spec, the materials are the likes of isooctane and toluene. All that being said, there are really 2 issues: Octane and lead content (for valve seat protection).
Octane: Aviation gasoline, by spec, must be at least 100 MON. There is no spec for RON, but data indicates about 104 or 105 is typical. The sensitivity (RON-MON) is dependent on the composition, and since avgas is made of only a few similar components, its sensitivity is small. Autogas, on the other hand, is made of many components and its sensitivity is around 10 numbers. Sooo, a 93 octane autogas could have a RON of about 98, and a MON of around 88. Avgas is usually around 102-104 MON (never as high as 110). Now if we run the numbers, we get a mixed RON of about 101, and a MON of about 106, or a (R+M)/2 of about 103. Rejetting of carburetors may be needed because the composition of the fuels will be changed, fuel injection systems should compensate almost immediately.
Lead: Avgas can contain 2.0 milliliters of TEL per gallon (maximum in 100LL). Therefore the avgas seller requires a wing number to sell avgas since it's not allowed for highway use (and no road tax). And avgas contains a scavenger (ethylene dibromide) to prevent spark plug fouling from the lead salts that are made during combustion. In any case, it makes it impossible to use with oxygen sensors and catalysts.
An example of the specifications for 100LL is at
http://www.chevron.com/prodserv/fuels/bulletin/aviationfuel/9_ag_specsandtest.shtm
The web page shows lead in units of gms/liter, not gms/gallon.
