Note that contrary to what some may say (or write), you can indeed mix Dex-Cool and conventional coolant with no ill effects. However, if there is more than 10% of conventional coolant in the system this will reduce the concentration of the Carboxylate such that conventional coolants change intervals must be followed. In other words, if you mix it, you can't leave it in for 100,000 miles or 6 years, but instead should treat a mix just like conventional coolant and change it every year until the concentration of Dex-Cool is over 90%.

Can DEXCOOL organic acid antifreeze be mixed with ethylene glycol antifreeze?
It is ethylene glycol based antifreeze! The concern with mixing comes from the fact that there are very different chemical inhibitor packages in use. Most leading technologies will work very well when used as intended, typically at 50% in good quality water. If the coolants become mixed with DEXCOOL, 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.