Fred,

Lets leave the octane discussion aside, for the moment.

Now, please excuse me if I break the wind on this fuels discussion,  but how
in the world are you going to keep auto gas from vapor locking going from
12,000 to FL 250 on an August day?  It is not easy to that, now, even with
100LL.

And if you manage to do that on, say, Chevron premium auto fuel, are you
going to do the same at the next airport with fuel refined in Wynnewood,
Oklahoma, to pick one example - - of many local sources of auto fuel.

Are you going to pressurize your fuel tanks ?  

Now, can we go back to reality on the discussion of fuels for aircraft
designed to operate in the flight levels?

Regards,  George

PS.  Auto fuels may work just fine for folks flying around below Pike's
Peak.