Turbo:
I must clarify my earlier statment. To "run cooler" was a vague way for me to have described my actual thoughts, as I'm sure the operating temperature of a gasoline engine is about the same as that of a diesel engine.
Diesel engines extract about ~40% of fuel combustion as kinetic energy compared to ~25% for a gasoline egine, which means that a diesel engine is only "throwing away" ~60% of the combustion energy as heat, compared to ~75% from a gasoline engine. (This is also why diesels take longer to warm up during winter; the metal of a gasoline engine will be at operating temperature sooner after startup than a that of a diesel engine.)
THUS our diesels require the cooling systems to dissapate much less heat energy per revolution than gasoline engines do. Think of the cooling system as the means by which the wasted heat energy is disposed of. This is why I don't think my 300D needs better coolant than a 300E. Furthermore, a 300D is a passenger vehicle, not a big rig; this is why I wouldn't use HD Diesel coolant.