You could run either distilled or deionized water. They share the same pH roughly (6.8-7.0) at neutral. Distilled is a more basic purification. Essentially boiled water (water turned to steam transfered to a clean resevoir back to water). Deionized is basicly filtered water. Special materials are used to remove impure ions (or ions of choice) from the water as it passes through the filters.
Most will argue distilled is more pure than deionized. Each has its use. Distilled removes more heavy minerals than deionized. Deionized removes more bacteria/chemicals. All will depend on the original water source.
Fact; Distilled water is readily available at most grocery stores at around $.80/gal. You could use either purification process and get the same results, pick your poison.
Bottom line; tap water, in general, is shitty water to put in your car. Most major cities and even some small towns put additional chemicals/stabilizers in the public resevoir to kill bacteria and neutralize odor, including chlorine and fluoride. Not to mention any additional minerals the water picks up along the way through the pipes and out of the tap.