Well, it's a fact that the tank will always accumulate some amount of moisture no matter, as compressed air condensates and creates moisture. What you have is just water in the line, I doubt it's oil. If you have oil in the line you will know as the oil level will periodically diminish and you will have oil in the pump head itself.
If you have a water and particulate filter on the unit now and go through them frequently then you may want to install a better filtering system like a "toilet paper roll" type of oil/water filter. And, you probably want to install a water/oil filter directly out of the outlet on the tank in addition to a second filter at the end of the line (hose) to your tool.
Dryer weather will produce less condensation of course, so the dryer weather the better.

This is about all you can do to minimize water in the line and in the tank. Also, make sure you drain the tank after EVERY use, this will keep your tank in good shape so rust doesn't cause a pinhole leak in the tank.

You can click on the below link to view the various Filters available, for your reference:
www.mastertoolrepair.com/air-compressor-...filters-c-25_60.html

Hope this helps!
- MTR