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The oil is supposed to be impure thats why there are additives, the oil most likely lasts 25,000mi because it does not have additives that wear out and detergents to remove crud that shows up as darker oil. Oil does not break down or wear out (unless excessively heated) it just gets dirty and needs to be reprocessed and have additives... added.
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"It's not rocket surgery!" -MetroMPG
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