Well, I have the MOGAS STC on my PA28-180. I had the engine rebuilt by
PenYan Aero and they will dishonor the warranty if MOGAS is used (per
the paperwork with my engine).

In talking with a few A&P/IA types, I found that 100LL contains more
TEL (Tetra Ethyl Lead) than I thought. And they tell me that this TEL
lubricates the valves.

Meanwhile I wrote PennYan Aero and asked the specific issues with
MOGAS, they've never replied (been over 2 months now).
Later,
Steve.T
PP ASEL/Instrument