Supermouse3: If you try and put MOGAS in an IO540 and run it at full power, it may produce the same power for a little bit before detonating, because of the lower octane rating and the high specific energy it will not run.
An IO-540 is just an IO-360 with two extra cylinders. An IO-360 is just a bigger brother to the O-320. The O-320 will run on mogas.
A few years ago there were plenty of O-320s being run on mogas on this side of the ditch with no adverse affects.
I cannot see why an O-540 or IO-540 would't run quite happily on mogas. In fact I suspect plenty of them have been run on mogas quite successfully.
Remember these are not high compression engines.