This paper describes an investigation of the effects of fluid properties on hydraulic motor efficiency through experimentation and modeling. Synthetic ester, straight-grade mineral oil, and VI improved hydraulic fluids were evaluated. Fluid properties, including viscosity, shear-stability, density, and traction coefficients, were characterized. A model for relating motor mechanical efficiency to fluid properties was developed. This model combines the viscosity parameter of Stribeck with the surface adsorption model of Michaelis–Menten. The results revealed that fluids with a low traction coefficient improved the low-speed efficiency of motors by transitioning out of the boundary lubrication region at a lower Stribeck number.