what mileage you doing? if you only doing little mileage, its probably condensation as gibby said. the engine isnt being run long enough to get rid of this, so it stays in the system. clean all the breather system out, then try going for long runs to try and clear it.
oh, and magnetec is crap. it breaks down a lot quicker than most oils on the market. use mobil/esso oils. much higher quality. if you wanna push the boat out, use mobil 1, 0w/40w.