i use petron radiator coolant its cheaper
in the service manual it says that use distilled water (which i personally use mineral water/distilled drinking water)
i think the reason for this is tap faucet water tends to have lots of residues deposits and chemicals which forms
a gummy or solid deposits when its heated.
try to check the inside of your kettle you will see lots of solid deposits there...
the same thing with our radiator, if you use tap water, as time goes by it forms a solid deposit which will clogged up some of the radiator small passages.
though it will take time to form a solid deposit in your radiator,
its better to use mineral dringking water or distilled drinking water to our radiator to avoid the said problem in the long run.
just remember, what you want to your body... apply it also to your engine

hehehe