One of my Runners had a problem like this last winter. He seemed smart enough to stay out of the pool, but wouldn't get out of the rain and would eventually be soaked through to the skin so I ended up drying him with a hairdryer on a few occasions and tried to make sure he was locked in out of the weather anytime it was bad. There is apparently a condition called 'wet feather' which can be caused by a variety of things which result in poor health, or, as you suggested, there can be a problem with the oil gland. I started adding brewers yeast to the feed and made an effort to get all the ducks free ranging for at least a part of each day and he seemed to improve right away. He was just so skittish when I first got him that I had been afraid to let him out of his pen and I'm not sure if that contributed to him being in poorer condition or not. With an improved diet and more exercise he started looking healthier and happier, he started preening more, and his feathers came back in in much better condition after his next molt (though they continued to get soaked up until that first molt). And, when he got his new feathers in, he started using the pool again with the other ducks, and I haven't had a problem since. I thought it was quite interesting that he seem to instinctively know that the pool was a bad idea.