What is in your radiator?

[SSLance]

What are those of you that track day or autocross your cars regularly running in your radiator?

I know that distilled water with some sort of water wetter is preferred for a couple of different reasons in cars that see track time, but I'm curious how many of you that also drive your cars on the street or live in inclimate weather run an antifreeze mixture in yours?

I have to remove and replace a 30 year old heater core that failed a couple of weeks ago, so I'll have my cooling system open and this got me to thinking what I should do since I plan on several more track days next season. First thing I'm going to do is test my coolant for electrolysis to see if that's what made my heater core fail or if it was just age.