i think if your looking for oil for racing diesel, use 5w40 or 15w40. syn or dino your choice. in a race gas engine the rotation assy can live with loose bearing clearence and use high viscosity oil to make up for proper clearences, but a diesel engine the high(operating, consider static and operating compression; static plus airbox pressure) the ignition under high pressure will shear the oil out of the bearings and pound them flat, leading to total loss of oil pressure, engine failure. i was always taught to use minimum viscosity to get desired oil pressure, saves engine from eating itself away. thicker oil will cause components to fight and use more HP to move, drag racing might get away with this, but i think you might get longevity and sustainable HP from an engine then loosing it up. i have a customer that gets his boat engines rebuilt every couple years, the rebuilders have him use 20w50, his engines start to run crappy and start to smoke towards end of 1st season. JMHO concerning oil cooling, having engine loose oil will flow quickly thru engine, same as not running a thermostat(flow control) in coolant system, oil won't have time to pick-up or give off heat from contact surfaces in engine. oil is primary coolant in engine that distributes heat to more surface to allow outside coolant(air or water) to pick-up and transfer from engine.
