In all honesty, you should use distilled water. The reason for this is that there are almost no impurites in it. A lot of the crap that comes out of your radiator is the sediment in tap water. Have you ever boiled a pot of water totally dry? All the crap that is left over in the bottom of the pan is what's going to end up coating your heater core and radiator. It's only a buck or so a gallon. I'd just buy it.