Moisture in ANY vented container will accumulate as the ambient temperatures swing. (think of water-droplets on the outside of a cold beverage...only the droplets are on the INSIDE)
This is why it is recommended to keep tanks FULL during storage. (less internal surface-area in contact with air)
There are diesel-fuel additives which address this issue. There are two distinct approaches:
- Some of them make the water precipitate OUT of the fuel
- Get trapped in lowest part of system/filters
- Some of them make the water suspend in the fuel
- Pass thru the filters out the exhaust
Bottom line: The environment in which the equipment exists will affect how much water accumulates in the vented enclosures (fuel, hydraulic, steering, xMission...etc)