The ASTM D7721 standard practice for determining the effect of fluid selection on hydraulic system performance was applied to the study of two excavators and a hydraulic dynamometer. Six fully formulated fluids spanning two viscosity grades and three viscosity index (VI) levels were evaluated using the dynamometer. Four of these fluids were also evaluated in the excavators. A common monograde ISO VG 46 hydraulic oil served as the reference fluid in both studies. In dynamometer testing, high-VI, low-traction fluids reduced hydraulic motor torque losses by 9 to 35 N·m. In excavator trials, multigrade hydraulic fluids shortened the average bucket filling time by 5.1% and increased the average bucket payload by 2.8%. Based on the physical properties of the test fluids and fundamental principles of fluid mechanics, the observed performance gains are believed to be the result of reduced viscous shear, boundary friction and traction losses. The results presented explore correlations between fluid properties and hydraulic system performance, enabling formulation strategies that can help reduce parasitic losses and improve machine productivity.