STC for mogas in O-235

Premium mogas is still much cheaper than 100LL. If I had good
availablity of both fuels I would probably rin a mix of 100LL and
mogas-the effect of the TEL in the avgas will boost the mogas's octane
rating much higher than that of each fuel separately, even with as
little as 10% 100LL.

We used to do that with both O-320s and O-200s, and both gave
trouble on either straight mogas or the mix. Unleaded auto fuel is
significantly different from Avgas, and even getting the lead level up
to 0.5 ml/l with a 75/25 mix (to make it the same as 80 octane), we had
valve sticking problems. The O-200 was particularly troublesome.
Several engine overhaulers told us they had customers who had trouble
with mogas or mixes, and they had seen a bit more valve wear in those
engines.
Apparently unleaded auto fuels are refined differently and have a
significantly different composition that older engines don't like. It's
"drier," as one guy put it. When we went back to straight Avgas the
problems went away.
Lycoming won't warranty their engines if mogas is used in them.

Dan