I need to flush my cooling system and fill it with straight water + Redline Water Wetter for my upcoming track day. I've already bought the Water Wetter - and went looking for some distilled water - but haven't been able to find any yet. Wal-Mart usually caries it, but is currently sold out. The closest thing I saw was some "purified" spring water. The label said it was purified by reverse osmosis. Is that safe to put in my cooling system - or should I hold out until I can find some distilled water?