The best suspect is someone added dye to the oil in the past to help in finding the location of a leak. When an engine takes five quarts of oil during a change, it actually holds about seven quarts. Two quarts in the pump and passages never drains out. It could take a dozen oil changes before all the dye is gone.
If my guess is right, the old oil will turn bright yellow if you shine a black light on it. Many auto parts stores have tools to rent or borrow. Stop in there and ask them to briefly shine the light on the bottom of your "oil" cap.
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Tuesday, August 1st, 2017 AT 6:32 PM