What type of coolant does everyone use?

One big thing that many folks miss is that in some areas, WATER is simply garbage. (My years in San Diego.) If you use a conventional coolant like Prestone, use "purified" store bought water, distilled if you can get it, instead of tap water.

It depends on the total hardness (calcium/magnesium ) of the water. I have soft water where I live so tap water is fine.