Suitable coolant / Corrosion inhibitor.

[rob love]

Antifreeze is cheap, and has all the inhibitors and lubricants required. I say use that.

Antifreeze does require to be changed every 5 years or so as it will get corrosive over time. I have dealt with very low mileage vehicles but with very old antifreeze, and the results weren't pretty.

Also, something a little newer is to test and adjust the PH level of your antifreeze. This has been available since the 80s as I recall, and isimportant, especially with the dissimilar metals of aluminum and steel.