While thicker oils impart a stronger film strength, remember that the oil needs to pass through very tight tolerances. If the oil is too thick, it may not flow fast enough and could starve the engine of oil. This increases wear at start-up, wastes energy pumping, reduces fuel efficiency, and can cause the oil to lose its ability to transfer heat. An increase in operating temperature can lead to a chemical breakdown and cause harmful sludge and deposits to form.
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