I would think if there was enough water in the system to cause problems with icing, you would notice it by the color and texture of the oil even when it's above freezing. Water and oil mixture gets ugly fast. Assuming (on my part) water is not the problem, I'd lean more towards filter restriction or the oil itself is a little too thick for cold temps. If the filter hasn't been changed in a while, I would replace that first and see if it helps at all. Maybe go out on a cold morning, pull the hydraulic dipstick and look at the oil. Does it look right? Does it drip off the stick at all, or just hang there?