A 30% ethylene glycol (common antifreeze)/distilled water solution will be about 3 times thicker than just water at 40 degrees F, but lower the freezing point of the solution to about 2 degrees F. That viscosity might sound like a lot, but you'd be suprised just how overly powerful the average impeller is; those over-the-counter w.c. liquids are commonly thicker than this because they also have all sorts of extraneous anti-corrosion and cavitation agents.
As long as your pumping enough heat into the system (i.e. never turning off you computer) even if you put your radiator outside the viscosity will never reach a high enough level to turn everything into slush because of low fluid transfer. I've put my rad outside here in Wisconsin during the winter (temp was around 5 degrees F) and didn't have any issues will poor flow.
Plus, the hotter the liquid gets the less thick it becomes, so overclocking in many ways becomes a requirement
link for info if your living in the arctic and want to try some ethylene glycol superclocking: http://www.engineeringtoolbox.com/et...col-d_146.html