The coolant that needs to be tested every six months is green and needs extenders added regularly. The Fleetguard ES Compleat OAT is blue and may need ES Extender (or charged filter in older engines) added every 150,000 miles, Cummins Irvine recommends testing every two years, though reportedly no additive is usually needed.
My engine came green and I changed it a few years back to the blue ES Compleat (dunno why they chose to spell it that way). It can be purchased undiluted or diluted.
There have been several very good discussions about the differences in fluid types and their pros and cons in Motorhome Magazine and Family Motor Coaching recently. Check their online editions.
Here is Fleetguard's ditty on it: ES Compleat?