Rotary shaft seals are used to seal flooded, partially flooded or splashed shaft outlets from machine housings, making them essential components in many technical applications. These highly complex tribological systems consist of not only the sealing ring, but also the shaft counterface, the fluid to be sealed, and the ambient fluid. Consequently, the shaft counterface significantly influences the tribological system, and, therefore, the functionality of the seal. As well as affecting the friction and wear of the sealing system, shaft lead is often responsible for leakage and, consequently, system failure. While these effects have traditionally been investigated through experimentation, this study introduces a numerical approach to analyse the impact of the shaft counterface on the lubrication and sealing mechanisms of rotary shaft seals. The results reveal correlations between surface parameters and the functionality of the sealing system, providing valuable insights for deriving more suitable tolerance limits for shaft lead in the future.