Electric Submersible Pumps (ESP’s) are multistage pump arrangements used in offshore petroleum production. Most of their applications are subject to viscous oil pumping, which causes performance degradation with respect to the regular service with water and changes some characteristics related to the flow dynamics inside the pump. The purpose of this work is to use CFD to investigate numerically the flow in a semi-axial type ESP with three stages operating with fluids of different viscosities. Both design and off-design flow rates are simulated, as well as different impeller rotation speeds. Head curves of the ESP for these cases are compared with experimental data and show good agreement. The importance of considering more than a single stage when studying ESP’s is discussed. The flow fields inside the pump channels for different operating conditions are compared, showing for instance that the flow is not always blade-oriented at the best efficiency point for service with fluids more viscous than water. The effect of the fluid viscosity and the rotation speed on the performance degradation is also explored. In addition, dimensional analysis is used in favor of a better understanding on how the pump performance degrades when working out of the design figure.