The performance of various SAE 15W-40 and SAE 30. API CD/SF and API CE/SG oils was compared in on-highway vehicles with different diesel engines. The API CE/SG oils provided superior oil consumption control, better engine cleanliness, and less engine wear than the API CD/SF oils. Among API CE oils, a 1% sulfated ash formulation provided improved oil consumption and carbon deposit control compared to a 1.3% ash oil. One vehicle was extended to slightly beyond 1,000,000 miles, demonstrating that high quality, properly formulated API CE/SG oils can extend the useful life of an engine to overhaul.