With five or six ducks under your belt, are you using a fleshing wheel? If you are getting oil on your feathers, after you have washed and dried, I would have to assume it came from the skin itself. While you are mounting your bird, running wires, and sewing it up, it's probably laying on it's back, and you are probably pressing oil from the skin, through the skin to it's feathers. Flesh better, and use a soft towel or blanket under your bird when you are mounting, and this should fix the issue on your next bird. I really want to stress the fleshing thing though, because this is ulimately the problem.