Prandlt number=viscosity*Cp/Thermal conductivity
Prandlt, Nusselt, reynolds, schmidt, as well as many other numbers interact algebraicly
Heat transfer increases proportionally with Reynolds number, but also with viscosity.
its a tricky topic as to which to use, distilled water or antifreeze. the distilled water flows faster because it has a lower viscosity. the lower viscosity raises the reynolds number of the fluid across the waterblock(hydrolic diameter is associated with the geometry of the waterblock). reynolds number increases inversly with viscosity, but heat transfer increases directly with viscosity and reynolds number. Antifreeze, although handicaped by its lower reynolds number, it's higher viscosity causes more friction with the radiator and tubing walls causing greater heat transfer out of the system.
it is quite complicated... to learn more, google fluid mechanics, convection heat transfer, reynolds number, nusselt number, prandtl number... it goes on.
the Cp, or specific heat capacity, of water is higher than that of antifreeze (ethylene glycol/water mixture) and so the water is capable of carrying more heat in it with less of a temperature change of the fluid. and as we all know, heat transfer is driven by temperature differences! this means that you may need a larger reservior if you are going to use antifreeze.
as you can see, the same things that advantage one in a certain respect will also detract from it in another. it is quite a design problem... i gotta stop typing and get back to work... designing heat exchangers, curiously enough