This paper describes a laboratory dynamometer/engine test procedure for acquiring fuel consumption data under a variety of operating conditions. A unique part of this procedure is the evaluation of fuel consumption at two jacket temperatures. This permits an evaluation of changes in viscosity without changing the oil. The ratio of fuel consumption at low temperature to that at high temperature has been defined as Viscosity Sensitivity Index (VSI). If fuel consumption decreases with increased temperature (neg. VSI), a large hydrodynamic lubrication component is in evidence. If fuel consumption increases with increased temperature (pos. VSI), boundary lubrication has increased. By this technique, engine condition and oil effects such as relative film thickness, apparent viscosity, and friction modifier effectivness can be evaluated.