Hello Pete:

I think the idea behind running the airplane once a week long enough to heat the oil into the green arc is to evaporate any moisture that has accumulated in the engine, and also to put a fresh coating of oil on all the moving parts.  As the engine sits, moisture condenses inside the crankcase, and the coating of oil that protects against corrosion thins out.  When I was a new aircraft owner, I was told to do that at least once a month, but then I live in a relatively dry part of Southern California.

Jon