As stated in # 11 above, the clean element pressure drop is used to determine filter selection. This pressure drop is derived from a calculation, which takes into account the pressure drop due to the empty housing and the pressure drop due to the element. These are combined to determine the total filter clean element pressure drop. This calculation should be done for both the operating temperature and for cold start conditions. Instructions on how to calculate this pressure drop are located on the last page of every filter tech data sheet (also referred to as the sales brochure). As a minimum, you will need to know the maximum operating pressure, the flowrate, the oil viscosity (SUS), and the desired micron rating (or desired cleanliness class or critical component in the system).