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The effect of viscosity index improver chemistry on the low temperature performance of engine oils in on-highway trucks must be understood in order to formulate oils for this application. Laboratory data, both from motored and fired heavy duty diesel engines, has been recently published, but no actual field performance in heavy duty diesel engines has been published. This paper reports on a testing program designed to compare the performance of three different VI improvers and two commercial oils under field and laboratory conditions. The test program was designed to determine if different VI improvers (1) affect engine oil pumpability as perceived by the operator, (2) satisfy the lubrication requirements of the engine, and (3) determine if different oil formulations perform the same in different engine designs. The testing program also addressed the correlation between laboratory and field testing of lubricant pumpability.