First question is diesel
Second question is can you actually empty the tank?
While I don't like the idea of a full tank of diesel sitting thier, it's not to big a deal as long as there is no water
already in the tank to grow things. Gasoline, especially with ethanol will go stale and the ethanol tends to attract water
Ideally, completely empty and dry is the way to go. Unfortuantely, most manufacturers don't provide a convientent way to drain the tanks
and partially empty is probably worse than full. We drain our tanks
in the fall and check they are still empty in the spring before filling up.
- Totally empty and condensation tends not to form as there is no heat sink to create a temperature differential.
- Totally full, there is little air to form condensation so water in the fuel is less of an issue.
- Partly full, the heat sink of the liquid helps form condensation and then it finds it's way to the bottom of the liquid where it can't evaporate.
Of course, I think the condensation theory is largely a myth. If you look at the amount of air that enters and leaves the tank over the course of a day, not a lot of moisture ever enters over the course of an entire winter. More likely it is leaks
fittings allowing rain and snow melt to find thier way into the tank. A full tank of gasonline may mask this as ethonal can absorb a bit of water before phase seperation occurs, so minor leaks
that only let in a small amount may not cause issues. On diesels, the water seperator may take care of a few ounces with little fanfare.