This is from
Sydney Camm and the Hurricane describing the difference made when the Merlin changed from 100% glycol to 30% Glycol, 70% H20
the lower operating temperature of the engine on this mix - also used by the Merlin XII - helped TBO and reliability. In theory the CLOD Merlin XII should not be overheating as much as it is right now. I'm not sure about the temperature difference of 70°C as quoted so I'll dig a little more.