Detailed measurements of dynamic and steady-state drop size distributions were performed in liquid-liquid systems with water and different silicone oils using an in situ endoscope technique in a discontinuously operated stirred tank. The oils used as the dispersed phase vary in dynamic viscosity, while density and interfacial tension are nearly constant. The performance of a population balance model for partially mobile interfaces and its ability to predict steady-state and dynamic Sauter mean diameters and the complete drop size distributions as a function of dispersed phase viscosity and different process parameters are evaluated. Furthermore, the impact of different daughter drop size distribution shapes on simulation results is quantified.