Not from an aviation background but automotive, oil is cheap, engines are expensive. I'd go with B2N2 and change the oil after and before the winter, it's not expensive. Blow by from a rich mixture will thin the oil quite rapidly.
Don't know if any regs apply, but if I was leaving an engine outside/in the cold for an extended period I would whip out the plugs and squirt in some oil, have a fresh oil filter and oil in it, consider using an old set of plugs in it instead of the new ones, keep new set at home and refit in the spring.(lets face it if your not using them all the plugs are doing is keeping out the water)
In the spring whip out the old plugs and spin it over a few times to expel oil etc. put in new plugs, then run it up to temp, then drain out oil and put nice new stuff in. Oil does a lot of weird things when left alone, nasty acids etc.
If you are only going to change the oil once, it's better to put the new stuff in in the autumn, then use it all summer, than leave in the stuff you've you've used all summer then leave it in the engine all winter. Don't leave old oil in an unused engine.