A. Either ethylene glycol or propylene glycol. Ethylene glycol is slightly more effective than propylene glycol at a 50:50 coolant to water mixture. On the other hand, propylene glycol had lower toxicity to animals, and can handle a higher heat level—up to 350 degrees Fahrenheit. In case you are interested, almost any antifreeze is made up of 95 percent glycol, 2 to 3 percent water, 2 to 3 percent inhibitors, a splash of antifoam and a trace of dye.