Rich I do understand your argument about equilibrium and saturation, but oil does not expand and contract with temperature (as much) as air does. The air will condense nightly and with temperature drops. The oil will saturate when warm and hold less water when cold, so there will also be some condensation but I think the action is much less with a full tank vs. an empty one.

Water in an empty tank can evaporate, but water that settles beneath oil will be trapped. But once a tank has been used there is no such thing as an empty tank unless you get in there with rags and soak up all the residual fuel. I'll stick with a full tank in winter unless you can convince me otherwise. It's been OK so far.