Jay Honeck wrote:

Right. Which makes me wonder why they don't simply come out with an STC
that specifies the use of higher octane auto gas (as in the example, above),
rather than simply not having a mogas STC for the high-compression O-540.

There is more about avgas than just the name. Lower vapour pressure,
lead, generally stricter specifications, and certainly a lot more that I
don't know.

Stefan