Oil sands are unconventional oil that consist of bitumen, water, and sands. Depending on the location of oil well the viscosity of bitumen is different. Also, during SAGD process there is a difference in the viscosity of bitumen according to the ratio of diluent. In this study, the performance of FWKO was analyzed numerically by changing the viscosity of oil. At the inlet the flow rate of oil was set to 10 BPD and water was set to 18 BPD. VOF was set as an multiphase analysis model and transient analysis was performed. The viscosity of oil was set to 6 cases: 100 cP, 250 cP, 500 cP, 750 cP, 900 cP, and 1000 cP. The performance of FWKO was measured as efficiency that is the ratio of inflow oil mass flow rate to the oil mass flow rate at the oil outlet, and mass flow rate of oil at the oil outlet. The results showed that the efficiency and mass flow rate of oil at the oil outlet increased as the viscosity of oil increased. But from 900 cP, mass flow rate of oil at the oil outlet and efficiency decreased. At 750 cP the mass flow rate of oil at the oil outlet and efficiency reached maximum of 0.01577 kg/s and 92.2%.