You need a copy of Lycomings Key Reprints in your life.
To my knowledge this is the 'official line' taken from the Key Reprints, found at:
Key Reprints - Lycoming
''Lycoming does not permit the use of any fuel other than those specified in our latest edition of Service Instruction No. 1070. Although Supplemental Type Certificates (STC) now make the use of automotive fuel, which meets minimum specified standards, legal for use in some aircraft, reciprocating engine manufacturers and most major oil companies do not approve. While it is true that octane levels appear adequate, these organizations are of the opinion that the varying quality control standards applicable to automobile gasoline produce undue risk when it is used in aircraft. Several specific reasons are given for the non-approval of automobile fuel:
1. Its use reduces safety. Although an operator may find that the engine runs well on a specific grade of auto fuel, there is no assurance that fuel from the same tank will be of the same quality when purchased the next time. Risk is increased.
2. Its use can void warranty, or result in cancellation of the owner’s insurance.
3. The storage characteristics of automotive fuel are less desirable in comparison with the good storage characteristics of aviation gasoline. After several months, stored automotive fuel may suffer loss of octane rating, and tends to deteriorate into hard starting, along with forming gum deposits that cause sticking exhaust and intake valves, and fuel metering problems, resulting in rough running engines. The turnover of automotive fuel is so fast that long-lasting storage characteristics are not required.
4. The additives in automotive fuels are chemically different from those designed for aviation, and contain auxiliary scavengers which are very corrosive, and under continued use can lead to exhaust valve failures. They also cause rust and corrosion in the internal parts of the engine. The allowable additives for aviation gasoline are rigidly tested and controlled. There is no uniform control of additives in automotive gasoline. Many different additives are used, depending on the fuel manufacturer. For example, one fuel company adds a detergent to clean carburetors. This additive creates a significant increase in the affinity of the gasoline for water which can cause fuel filter icing problems in flight if outside temperatures are cold enough.
5. Automotive fuels have higher vapor pressures than aviation fuel. This can lead to vapor lock during flight because the fuel companies advise that automotive fuels can have double the vapor lock pressures of aviation gasoline, depending on the seasons of the year and the location because of climatic conditions. In addition, automotive fuel also increases the possibility of vapor lock on the ground with a warm engine on a hot day.
6. Although the fuel octane numbers shown on the pump of automotive and aircraft gasolines may be similar, the actual octane ratings are not comparable due to the different methods used to rate the two types of fuels. Furthermore, aviation gasolines have a lean and rich rating, i.e., 100/130, whereas motor gas is not tested for a rich rating.
7. Automotive fuel used in an aircraft engine may lead to destructive detonation or preignition and potential engine failure at high power conditions.
8. Please review the Mo-Gas fuel requirements in your state or destination.
SUMMARY:Auto fuel is now being used as a substitute for Grade 80 aviation gasoline under STCs issued by the FAA. Most major oil companies and engine manufacturers continue to recommend that aircraft piston engines be operated only on aviation gasoline. Deterioration of engine and fuel system parts have been reported in aircraft using auto fuel. Operators should consider the added risk of using auto fuel in aircraft. Remember — a pilot can’t pull over to the side of the road when fuel creates a problem with the engine.''
...anyone remember the trouble Tescos encountered a few years back? Infact, google 'Tesco fuel problems' and it auto suggests 2012, 2011, 2010....
http://www.pprune.org/jet-blast/478608-tesco-fuel.html
Regardless of whether you decide to pay the '$750 STC' (I personally wouldn't even think about it), I would strongly suggest reading the Key Reprints cover to cover anyway, as it is brimmed full of info on your engines.