Black oil shows the detergent component is doing it's job. It's supposed to come out black,it has picked up the carbon (from blow-by) and is holding it in suspension so it can be removed. If the oil came out clean after a longish period of use that would be bad.
something that was drummed into us apprentices at GG&H - the five functions of oil are to lubricate, seal, cool, prevent corrosion and CLEAN.
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