Now consider this:
Perhaps the MMO is just slowing the oil from crusting up from the heat. Modern oils aren't made for aircooled flatties. The additives burn into hard soot.
There will never be enough flow of lube to cool a flattie's valve stems.
Nitrided stems and soft cast iron theory assumes next to no flow. Modern OHV's even have seals to prevent flow, lest carbonization fills the gap, resulting in sticking and wear.
MM Oil is the one lube that carbonizes the least. But you cannot run pure MMO!
Conventional oils with modern additives will carbonize and stick a properly fitted valve at flatty temperatures.
(Inspite of what Shell says, Aeroshell is probably the best motor oil for such an application, as it is ashless.)
And no doubt moderate use of MMO can only help.(You know when you have added too much when your plugs foul out!)
I have no qualms about using MMO as a safety net, but it is just that, and only that.
It is not a cure for the causes of elevated combustion temperatures. Only common sense does that.