The study of Flow Factors variation during run-in is carried out. A method to overtake the limitation of the input matrix dimension in the simulation program is developed. In particular, the software, which is used for the simulation of Flow Factors (Tribo-x), takes a matrix of 300 times 300 points as input, while the microscope measurement points are many more. So, the PSD of the surface is studied in order to find a relevant distance between two measurement points for the construction of the input matrix. Furthermore, the use of a Gaussian filter to reduce the measurement noise is discussed. At the end, all the results are presented and the behavior changes under different loads, temperatures and times are compared.