A 1995, 276kW (370 BHP) diesel engine was instrumented to provide measurements of temperature and oil pressure in five critical engine areas: the oil filter, the main gallery, the rocker shaft, the cam roller follower and the turbocharger. In addition, oil flow measurements were completed at the turbocharger. Using a proprietary test procedure, four different commercial SAE 15W-40 formulations were evaluated at -15 °C.Significant differences were observed between the test lubricants in terms of flow rate and time for establishing pressure in critical areas of the engine. In addition, with one oil, as a result of the high pressure drop in the filter, the bypass valve stayed open for several minutes, leaving unfiltered oil circulating in the engine. Our findings show that the MRV TP-1 viscosity at -20 °C or -25 °C is not capable of predicting all aspects of pumping performance of lubricants in a modern heavy duty engine.