RClark,
For a how-to, see my article in TDR #35, pages 76-81.
Ignore the section on using voltage as a method to test coolant; it doesn't work.
My choice of coolant is Caterpillar Extended Life Coolant, part number 101-2844. This is pre-mixed; you can also buy concentrate for about twice the price. This coolant is good for 300,000 miles or 3 years, whichever comes first. At that point, you add the appropriate amount of Caterpillar Extender and go another 3 years. After 6 years, you have to change the coolant.
The choice of coolant is not critical, as long as it's an ethylene glycol type and meets Cummins' specifications. The main difference from a user's perspective is how often you have to change it. The Caterpillar with extender lasts longer than anything else I've found. The reason I bought Caterpillar is because they were the only people who had the extender in stock.
Loren