For heat transfer, for liquids water is almost magic. No low cost material can match it. We add ethylene glycol, although it reduces the net thermal conductivity, for both protection at the low end, and more importantly, allowing the engine to run hotter without the coolant boiling.
Remember from your thermodynamics class, efficiency is gained as a function of the temperature delta between the hot and cold reservoirs, and the push over the years for both increased mileage and reduced pollutants has seen incremental increases in operating temperatures. Performance in jet engines over the past seventy years have largely come with advances in metallurgy and cooling tricks in the turbine blades that allowed an upward march of the Forward Turbine Inlet Temperature. In fighter planes, pilots are allowed to mess with this in an emergency, (e.g., do this one time to get away with your life - but the engine has to be rebuilt when you land), and while I don't have any recent data, in early days, a few pilots would overuse that feature, and lose the airplane when the engine came apart. If they were doing it supersonic speed, they lost their lives in the bargain.
