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A literature search to identify deleterious effects of using re-refined oil did not disclose any validated occurrences. Significant engine testing using re-refined lubricating oil is reported and no cases were discovered in which engine operation was affected negatively by the use of re-refined oil. The American Petroleum Institute (API) allows the use of re-refined base stock oils in the blending of end use lubricants. Based on oil sample testing performed in this research as well as other authoritative sources, it was determined that no significant chemical or physical differences exist between re-refined and virgin oils. Differences noted in this research were related to higher levels of poly-nuclear aromatics (PNA's) in the re-refined oil. PNA's are formed due to the extreme conditions of temperature and pressure during operation of an internal combustion engine.