After Run Oil
Though most people blame the methanol for attracting moisture I suspect that condensation is the major reason for water in the crankcase. Reason is that I have removed the backplate from engines right after running, even when not run dry much of the fuel had evaporated prior to removing the backplate. Without the methanol even the oil in our fuel will not dissolve with water, the water sinks to the bottom of the crankcase, if the engine is pointed up the water will stay on the backplate without much corrosion, if the plane is pointed down the water will go toward the bearings causing them to rust.
Just an observation that many people who have problems with rust store there planes or engines with the front pointing down.