A diesel engine fuel economy test has been designed to measure the BSFC effects of HD oils of different viscosity grades and additive composition. A Cummins NTCC 300 diesel engine was run for 15 hours on each oil at many steady state speed and load conditions, and multiple measurements were collected at each condition. Analysis of the data confirmed statistically significant differences. Based on three different viscosity grade oils (SAE 10W-30, 50, and 15W-40), a 1% fuel consumption improvement was measured for each one cSt reduction in viscosity. Oil compositional differences of four commercial SF/CD 15W-40 oils were found to affect fuel consumption, and the benefit of friction modifiers on BSFC was measured.