Bmw750Li

Not using distilled water is just dumb. Distilled water has no contaminates in it. Perhaps you would be better off buying coolant that is pre-mixed with water 50/50?

While pure water will work in a pinch IF temperatures never drop below freezing, (a) it can destroy the engine if it does freeze and (b) it doesn't contain the anti-corrosion additives which are common in modern antifreeze mixtures, so it probably isn't as good for your engine as the proper dilution of antifreeze.

Water contains minerals- when distilled, it leaves these minerals behind- the use of regular water will eventually leave your fancy aluminum radiator coated with mineral deposits-