Trebuchet03, the resistance to a heavier fluid is hydrodynamic drag. Since the engine parts push the oil out of the way as it turns, the heavier (greater resistance to flow) the oil, the greater resistance for the engine ... otherwise known as higher load.
Coefficient of friction is different. They must use fluids of equal weight (cSt - centistrokes) and then rub one surface against another (say, a small polished metal plate against a larger flat surface) and then measure the resistance.
That's just a guess ... but some SAE-type tests are actually that simple. Ever see a Timken bearing machine used in oil tests?
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--- Bror Jace
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