Most people I've talked to use tap water. I always use distilled water, just for the peace of mind if anything else.
Samilcar said:I'm not sure what you're trying to get at by differentiating a solution of table salt and water from what comes out of a water softener.
Water softeners work on a system of ion exchange. They do not work by simply mixing salt with water and serving you this salt water solution to drink. While water softeners do use salt (either Sodium Chloride or Potassium Chloride) in the ion exchange process, this salt IS NOT PRESENT in the treated water that comes out as a finished product. Any Sodium that is present in softened water isn't in the form of Sodium Chloride (salt), but as an elemental Sodium ion. A solution of Sodium Chloride and water would be very corrosive and would not be the best thing to put in your radiator. A solution of Sodium ions and water is NOT corrosive, and I would have no qualms about using it in a cooling system.