This paper describes the results of a series of tests on a heavy-duty truck diesel engine using conventional and low viscosity lubricants. The objectives were to explore the impact of reducing lubricant viscosity on wear, friction and fuel consumption. The radiotracing Thin Layer Activation method was used to make on-line measurements of wear at the cylinder liner, top piston ring, connecting rod small end bush and intake cam lobe. The engine was operated under a wide range of conditions (load, speed and temperature) and with lubricants of several different viscosity grades. Results indicate the relationship between lubricant viscosity and wear at four critical locations. Wear at other locations was assessed by analysis of wear metals and post test inspection.The fuel consumption was then measured on the same engine with the same lubricants. Results indicate the relationship between oil viscosity and fuel consumption under a wide range of operating conditions. Expected fuel consumption improvements over a typical drive cycle were calculated.Friction of the whole engine was calculated from measurements of cylinder pressure and brake torque, with two of the low viscosity oils and, in addition, a five stage motored friction teardown test was performed. Together these results were used to explore the relationship between lubricant viscosity and friction across a range of operating conditions.