Think about this .... do empty tanks magically fill with water from condensation? of course not....
I hate to argue with Rick, but in this case he is provably wrong. I have tested this specifically on different scales, from lab to thousands of gallons.
No, an empty tank would not fill with water. When dew falls from the roof it lands on the bottom and evaporate on the next nice day. What happens, however, is that the water falls through the oil and becomes trapped where it cannot evaporate. The process is slow and will not amount to more than an ounce per year, depending on tank size, climate, and the position of the tank in the boat. But an ounce can lead to a lot of corrosion and bio-growth issues.
CAT apparently has seen the same thing (from the link below)
Water can get into your fuel if itís mishandled by your fuel supplier. Most often, however, water gets into fuel tanks by condensation from the atmosphere. As the tank empties, air enters the tank. Water condenses on the walls and runs down the sides. The water never evaporates because it's heavier than fuel and goes to the bottom of the tank.
After this process is repeated several times you may have a significant amount of water in the bottom of your tank."
No valve; maintaining a clear vent is critical to safety.
The greatest risk is almost certainly your own fuel filler cap. In fact, any time there is more than a few ml of water, there has been a leak into the tank, either on the boat or in the distribution chain. But given that fact that, in general, boaters have water problems and truckers do not, suspect condensation.
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With a 20 gallon tank operated 100 hours/year, old fuel is not a big problem; I would fill the tank. I would tend to agree with Rick if the fuel was going to be kept more than 6 months. Condensation or not, fuel tends to darken and deteriorate over time.