Oilfield separator data ranging from light-oil conditions to heavy-oil conditions were incorporated into suitable two-phase and three-phase computational fluid dynamics (CFD) models to provide improved design criteria for separator design methods. The CFD simulation results revealed that the most important affecting parameters are vapor density and oil viscosity. In contrast with the classic design methods, noticeable residence times were required for liquid droplets to penetrate through the fluid interfaces. Moreover, it was indicated that the Abraham equation should be used instead of the Stokes' law in the liquid-liquid separation calculations. The velocity constraints caused by re-entrainment in horizontal separators were also studied and led to novel correlations.