I use Evans NPG coolant in my engine and have for over two years now. I like it for these reasons:
It WON't boil over, I've had coolant temps of 300 degrees with no problems.
It is not pressureized at all.
It won't freeze. At very low temps it turns into a gel and actually contracts.
It is more efficient than water /
antifreeze. I beleave it is one of the reasons that I can run pump gas at 12 to 1 compression.
This stuff is expensive but I have not had to buy much over two years as I reuse it.
Scott...