The factory service manual tells us to use only "soft (demineralized) water in the coolant mixture."

what is soft water, how would I get some, and most importantly, does it matter?

 

Just buy a gallon of distilled water at your local grocery store. It's basically the same thing. They are both a more "pure" form of H2O.

 

Yep demineralized or purified water is available at most grocery stores.

BTW you should use that water in anything that heats water to boiling points and you need to keep free of gunk, such as Irons (for clothes) humidifiers, radiators, etc.