IMHO Avoid that GM dexcool red stuff like the plague! It is crap, Ive heard that from a few sources and mechanics.

Trust green stuff, and change as per the manufacturers spec.

I would only use distilled water in an OPERATING cooling system. Flushing with tap water may not be that bad. Distilled Water is a solvent and therefore dissolves a suitable solute no different than tap water. The only difference in the tap water is that it potentially has a solute already dissolved into it, in which you are introducing into your various coolant passages.

Theresa the Toyota Terror-89 Toyota P/U (3.0L SR5, 5sp swapped in from auto, 3"B.L., 32X11.5X15 MT/R, 2.5" stainless magnaflow catback exhaust, leaky floor with cold draft @feet, FUBAR comptron now in glovebox)