I would think that grease would hinder the operation of the tube, not help it. Most grease will thicken up pretty good when it gets cold out. Why are you applying the grease in the first place? The only reason I can think of might be to prevent water from freezing in there. If that is the case, I would tend to use a light oil, not grease. Trouble is the oil will attract dirt to the tube. I'd try cleaning them out by running some yarn down the tube & pull it back & forth (maybe with some degreaser if you've already greased them) to clean them out.