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The inherent oxidation stability advantage that Group II stocks have for passing these tests is apparent in today’s engine oils. For example, Cummins and Chevron found that some commercially available engine oils consistently passed the MRV TP-1 after a 400-hour double-length Cummins M-11 test while others failed1 Subsequent analysis showed that the oils that passed were formulated with Group II. The oils that failed were formulated with Group I. A more recent study showed that Group I formulated oils stayed in grade only about a third as long as Group II (125 hours for Group I vs 400 hours for Group II) in heavy-duty engine oils formulated with the same non-optimized general offering package.