[f5fstop]

You can add water to dex-cool, so long as you keep a 50/50 mixture. Same as any type of anti-freeze, you can purchase the straight dex-cool, then mix with water for a 50/50 mixture. However, they do sell gallons of pre-mixed dex-cool/water.
I tried this stuff in my Vette, and maybe noticed a five degree temp decrease, but nothing I could every verify. Didn't do any controlled tests, just calculated a five degree drop due to normal driving habits and temps.