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Oil viscosity WILL impact fuel milage!!! Why do you think all of the car mfgs went ot 5W-30 and 10W-30 motor oil! I thought the original specification for Opels was 10W-40, BTW.... Synthetic oils offer more improvement in fuel milage.
The August issue of Circle Track even does a Dyno comparison to illustrate the increased HP from synthetics and lower viscosity... Interestingly enough, some of the HP increase comes from a reduction in HP consumed by the oil pump........
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