Oil pumpability in passenger car gasoline engines was well-characterized by an ASTM program and by individual researchers in the 1970's and early 1980's. Oil pumpability in diesel engines however, was not investigated to any significant extent until the mid-1980's. This study was initiated to define the performance of several commercial viscosity modifiers in different formulations containing 3 detergent-inhibitor (DI) additive packages and 4 basestock types. The test oils were run at -18°C (0°F) in a Cummins NTC-400 diesel engine. The results, when statistically analyzed, indicated that a new, second generation olefin copolymer (OCP) viscosity modifier had better performance than a first generation OCP and, furthermore, had performance equal to a polymethacrylate (PMA) viscosity modifier. The analysis also showed that one DI/base stock combination had a significant effect on performance.The apparent shear rate of the oil in the pump inlet tube was calculated from the oil pump flow rate measured at idle speed at low temperature and the pump inlet tube diameter. The shear rate and oil viscosity were used to estimate the shear stress in the pump inlet tube. The shear stress level of the engine is 56% higher than the Mini-Rotary Viscometer (MRV). Hence, the current MRV procedure is rheologically unsuitable to predict pumpability in a large diesel engine.A new device was developed for measuring the oil film thickness in the turbocharger bearing and noting the time when a full oil film is formed. Results indicate that a full oil film occurs almost immediately, well before any oil pressure is observed at the turbocharger inlet. Residual oil remaining in the bearing after shutdown may account for this observation. The oil film maintained its thickness both before, and after the first indication of oil pressure. More work is needed to study this effect.