Just so it's clear for other's reading this thread, there is nothing wrong with using auto coolant, and as most have anti corrosion additives included, they definitely should be considered for those who use mixed metals in their loop or those who want a little color in the coolant without staining their loop.
The only real issue, as has been mentioned, is that auto coolant does not have the same thermal transfer abilities as water, so your loop will run a few degrees hotter running auto coolant then if you were to run straight distilled water.
Saying that, most who still run auto coolant run it at pretty low mixes to improve thermal transfer anyhoo. And some also add water wetter to improve their autocoolant mix's thermal transfer ability to make it closer to the performance of distilled water alone.
At the end of the day, the only reason NOT to use auto coolant comes down to one simple question:
1. Are you chasing the bleeding edge overclock for daily use?
If you are not chasing the bleeding edge overclock for daily use, then a autocoolant mix will be perfectly fine and will cause you much less headaches then using distilled water and having to deal with cloudy loops, corrosion, gunk buildup, etc, and needing to flush the loop every few months.
If I can be bothered, I'll do a few runs with distilled water + silver kill coil vs different autocoolant/distilled water mixes to show that the temp difference is so negligable that the benefits of using autocoolant outweighs the negatives. From tests I did a few years ago, the difference was barely measurable - like ~2C at max load and pretty much identical at idle.
Edit: Just found THIS
shottout on bit-tech. Which shows the negligable difference in loop temps Edited by un-nefer - 5/27/11 at 1:14am