Propylene glycol, 1 gallon
Food-grade propylene glycol freezes at -12.9 degrees Celsius and mixtures of propylene glycol with water can remain liquid at temperatures as low as -52.8 degrees Celsius. The ability to operate at low temperatures allows glycol chillers to cool your wort more quickly than a water-based chiller. In effect, a smaller glycol chiller can do the job of a much larger water-based chiller.
In addition, the cold temperatures provided by glycol allow the chiller to respond to temperature spikes quickly. The rate of heat transfer is proportional to the difference between the temperature of the item being cooled and the temperature of the coolant. A low-temperature coolant can help your system return to its target temperature more quickly.
Be sure to check online sources for dosing rates. The total liquid volume of your chiller as well as the desired temperature are factors in determining the correct dosage of glycol to water. As a general rule, 1 part propylene glycol to 2 parts water is a "standard" mix.
NOTE: this glycol is inhibited, meaning it contains the necessary inhibitors to provide protection against corrosion of your system and glycol degradation.