Hydraulic auxiliary units account for a large part of the energy consumption of machine tools. In previous studies their energy consumption could be reduced through the use of efficient components and optimized operation strategies. Simultaneously, the selection of the hydraulic oil can have an impact on the energy consumption of a hydraulic system, as has been shown in test benches and in field tests. The mechanisms of energy losses in a hydraulic circuit are influenced not only by the hydraulic control strategy, but also by the viscosity of a hydraulic fluid, which indicates the fluid’s resistance to deformation. A too high viscosity can cause high friction losses and impair the pumpability of the fluid, while a too low viscosity promotes energy losses through leakage. However, neither an estimate for energy saving potentials of an oil change, nor suggestions for the selection of energy efficient oils exist for machine tools. Therefore, this paper examines the effect of different hydraulic oils on the energy demand of hydraulic units of metal-cutting machine tools.