The mechanical efficiency of hydraulic pumps is affected by the viscosity of the hydraulic fluid. Viscosity modifiers that thicken the fluid, therefore, play an important role in efficiency. Viscosity modifiers are believed to improve the mechanical efficiency of hydraulic systems partially by enabling formulation with lower molecular weight base oils. Here, this concept was directly tested in a pump dynamometer using mixtures of low traction synthetic poly(alphaolefin) base oils, bis(2-ethylhexyl) adipate ester, and poly(isobutylene). Lower viscosity fluids directly correlated to better mechanical efficiency but decreasing the viscosity of the synthetic base oil by adding viscosity modifier did not have the same effect. However, molecular dynamics simulations showed that solution viscosity was directly correlated to elongation of the polymer under shear which, together with calculations of the critical shear rate range in a pump, suggested ways of designing viscosity modifiers to achieve a specific viscosity profile that maximizes mechanical efficiency.