Originally Posted By: jongies3
They are the exact same viscosity at operating temperature, and the 5w will warm up faster which is a good thing for a diesel engine.
the 5W CAN'T warm up faster, as the 15W will have a higher viscosity DURING the warmup phase, waste more heat, and turn that heat into oil and coolant temperature...the 15W will generate more heat, and waste a tiny bit more fuel.
As to exact same viscosity at temperature, the HTHS between a 5W40 and 15W40 is around 10%, scarcely the same...
Originally Posted By: jongies3
Not to mention, it runs a few degrees cooler with the synthetic which I definitely like to see!
Is that because the 5W40 has a lower operational viscosity, and causes less heat to be generated in shear ?
Then how can it warm up faster (generate more heat) AND generate less heat ?