Depends on which Zerex. Zerex DEX-COOL is a fully licensed OAT-based DEX-COOL formula. Zerex G-05 uses hybrid corrosion protection - OAT with reduced silicate levels. DEX-COOL (and the various "dexclones") use OAT, alone, for corrosion protection. The differences between the various dexclones and genuine DEX-COOL boil (no pun intended...) down to the genuine stuff having been licensed by GM to use the DEX-COOL trademarked name and tested to meet the DEX-COOL specification. (By the way not all DEX-COOL licensed fluids use exactly the same chemistry - DEX-COOL is a performance specification, not a specific recipe. They do all have to be compatilbe, though.) The dexclones are every bit as compatible, but no licensing fees to GM have been paid. In the case of Prestone DEX-COOL in the silver-gray jugs and Prestone Extended Life in the familiar yellow jug, the only difference in the two fluids is that the licensed DEX-COOL is dyed orange, and the yellow jug fluid is dyed greenish yellow. In the case of the two Prestone fluids, both use exactly the same anti-corrosion chemistry. If your car was originally filled with DEX-COOL, and is still under warranty, stick with a genuine DEX-COOL for the duration of the warranty.
