I'd probably use a good quality car coolant, and dilute as directed with distilled water.
Coolants contain corrosion protectors to protect the radiator and heatsink, and contain lubricants to protect the pump. It really doesn't matter what type you use, as long as you flush the system out thoroughly first. They can slightly reduce cooling capacity - so don't use more than is directed - in fact, if you diluted it up at half-recommended strength that would probably be optimal.