Is phosphate. The lower cost green and/or yellow contain it. Even in low amounts it can contribute to lower pH during use and cause the formation of phosphoric acid. Copper, brass and iron could manage it fairly well but it's taboo for aluminum. Recall when radiators were made of brass?? Zerex states (although I never tested it in the labs I've worked) "phosphate free." The web site claims some sort of acid control - trade secret, no specific formula.

As for "compatible with all brands" I have heard mechanics dub red-orange GM brand dex cool, "DEATH cool." When mixed with some types of differing chemistry, it gels!! and WILL NOT FLOW!