About 60 years ago my dad told me that he sold old
oil back to some of the
oil companies. He said they would refine it and would come out with a product that was used where a finer grade of
oil was needed. Never thought about it until Patrick mentioned old oil being reused. Dad owned a Mobil station from right after WW2 until he passed i 1987. If I broke it he made me fix it. That's how I learned to wrench.