"RichH" wrote

Why store the oil in a tank with an atmospheric vent only to have it uptake
water (vapor), etc.?

Exactly.

Please confirm that I'm correct in the advice to others (some outside the
newsgroup) that there is no point in filling the tank unless it's filled up
into the vent so that the surface area of fuel available for water vapor
transfer is the cross section of the vent pipe and not the whole top of the
tank.

Since this is impractical in many vessels and impractical in most normal
operation, the whole topping up the tank thing is a crock.

I'm going to put a desiccant vapor trap on my sounding tube for the winter
which is easy to do and tape over the vent.

--
Roger Long