The effects of high-lead slag viscosity on gas−liquid mixing efficiency, splashing behavior, and furnace lining erosion were investigated in an oxygen bottom-blowing lead smelting process incorporating lead-containing waste materials. A multi-fluid volume of fluid (VOF) model, integrating experimentally determined slag viscosity values, was used to examine these interactions. The results indicated that optimal gas−liquid mixing occurred at viscosity values of 0.01 and 0.5 Pa·s, while minimal splashing was observed within the viscosity range of 0.1−0.25 Pa·s, corresponding to temperatures of approximately 1076−1100 °C. The regions surrounding the oxygen lances were most susceptible to erosion. Shear stress increased with an increase in melt viscosity, particularly rapidly in the viscosity range of 0.1−0.25 Pa·s. It was recommended that the melt viscosity should be maintained within 0.01−0.1 Pa·s to minimize erosion.