Quote:
Originally Posted by Bror Jace
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.
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This is why all manufacturers have switched to 5w20 weight oil as the lower engine load improves fuel economy which inturn makes a manufacturers average fuel efficiency for all vehicles produced higher.