The operating conditions of oil mist filters along with the filter media properties are the decisive factors for the evolution of separation efficiency and pressure drop during filter operation. Experimental studies were carried out to determine the evolution of clean gas concentration of oil mist filters with different media properties with regard to the oil transport mechanisms and its dependence on the operating conditions of the filters. The experiments were carried out using two types of glass fiber filter media, one oleophilic and one oleophobic. The effects of filter face velocity and oil loading rate were investigated for both filter media. The effect of media thickness was explored by varying the number of layers. It was found that filter overall separation efficiency can be correlated with the oil transport mechanisms analogously to the correlation to pressure drop proposed by the Film and-Channel model by Kampa et al. (2004). Decreasing velocities led to a stronger increase in overall penetration when liquid channels are formed in both types of media, which can be attributed to a loss of efficiency for droplets smaller than the MPPS in the size range of 0.1 mu m and below, while increasing the oil loading rate had the same effect, however not as distinct. Increasing filter thickness by adding filter layers also caused a stronger increase of penetration during this channel section. The formation of an oil film caused a drop in penetration. The effect of the oil film on penetration was found to be independent of loading rate, but not of velocity, due to its significance for inertial deposition. For highly efficient oleophilic filters, entrainment was found to be the dominant factor for clean gas concentration in steady state.