Good discussion. From a technical paper: "The heat exchange capacity of the EG (ethylene glycol)/water solutions is reduced with increasing EG content. Water remains the better heat exchange fluid compared to any mixture of EG and water. A compromise between the required freezing protection and heat exchange efficiency has to be made. The viscosity of the cooling fluid is also a factor in evaluating the overall heat exchange efficiency. Lower viscosity (better fluidity) will aid heat transport". From this, sounds like distilled water is best (with added rust and foam inhibitors, of course), anything else is either worse or magic. There's lots more, but I can hear the zzzzzzz's already.