A single screw expander (SSE) is a small to medium-sized volumetric expander of high performance, which can be used for energy recovery in vapor compression refrigeration (VCR) systems and expansion refrigeration in air compression refrigeration (ACR) systems. As an energy recovery device, expander operation improvement is helpful to improve the efficiency of the system. The expander, however, suffers from gas leakage losses and friction losses due to the viscosity of lubrication oil, where greater viscosity results in lesser gas leakage and more viscous friction. This study undertook an experiment to investigate six different viscosity lubrication oils (oil viscosity type from small to large: 10#, 22#, 32#, 46#, 68#, and 80#) under different inlet pressures (0.5–1.1 MPa) for clarifying the influence of lubrication oil viscosity on the operation of SSE. The results showed that increasing the kinematic viscosity of the lubrication oil could reduce leakage loss, which results in a maximum increment of volumetric efficiency of 23.06%, and with the use of 68# and 80# lubrication oils, volumetric efficiency was the highest at 92.7%. In contrast, reducing the kinematic viscosity of the lubrication oil could improve the shaft power. The maximum difference in shaft power between 10# and 80# lubrication oil was greater than 26%. The SSE had a superior shaft efficiency and a lower gas consumption rate by using 10# lubrication oil at an inlet pressure <0.55 MPa, 46# lubrication oil at an inlet pressure of 0.55–0.9 MPa, and 80# lubrication oil at an inlet pressure of 0.9–1.1 MPa.