If one have the choice between the same model plane with an IO-540 or an O-540 .. which route to go? IO could give better fuel burn in theory but the O has STC for MOGAS.

Mogas is too hard to find. Take the fuel burn. I'm looking for the IO. YMMV.

I can't remember the last time I saw mogas...

I like the IO...

I asked a similar question not long ago. This might be relevant to you:

https://www.backcountrypilot.org/forum/ ... -540-14749

pros / cons are pretty balanced.. i won't let it be a deciding factor for me.. I could get Ethanol free gas here in AZ, but have to haul it to the plane.

Really just wish a smaller engine plane would fit the 'mission' of wanting to carry bikes, dogs and go places..

In the marine world most pumps I have seen are MoGas because ethanol and marine engines and tanks do not mix well.

Seaplane operations: I vote unleaded MoGas. (O) 540 NOT (T), (G), (I) etc. etc...