I found this bit of info while I was searching around:

"Most leading technologies will work very well when used as intended, typically at 50% in good quality water. If the coolants become mixed with DexcoolŽ, however, one study showed a possible aluminum corrosion problem in certain situations. The other question is a concern for dilution of the protection packages. At what mix is the there too little of either inhibitor to protect the engine? As a precaution, both GM and Caterpillar instruct that contaminated systems must be maintained as if they contained only conventional coolant.
I would not recommend using DexcoolŽ in a vehicle that did not come from the factory with DexcoolŽ in the cooling system. It would be very difficult, if not impossible, to flush out all the conventional anti-freeze coolant from the cooling system of an older vehicle, and any conventional anti-freeze would contaminate the DexcoolŽ.
Compared to old-fashioned phosphated antifreeze, DexcoolŽ may be more stable and improve water pump life. Evaluations of the two technologies to compare their respective service lives have found them comparable. In fact, a Ford Motor Company study concluded that organic acid coolants do not offer any significant advantages for the consumer over current North American coolants. In a modern car with a well-maintained cooling system, current North American and OEM factory fill coolant corrosion protection can be extended far beyond previous expectations.
So the bottom line is this, if your car came from the factory with DexcoolŽ, use DexcoolŽ for replacement or to top off. If your car came from the factory with standard "green" antifreeze, use that for replacement or to toping off. Case in point, DexcoolŽ has been known to cause head gasket and water pump failure on some Ford OHC V-8's."
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