This part of ISO 13357 specifies a procedure for the evaluation of the filterability of dry lubricating oils, particularly those designed for hydraulic applications. The procedure only applies to mineral-based oils, since fluids manufactured from other materials (e.g. fire-resistant fluids) might not be compatible with the specified test membranes. The range of application has been evaluated with oils of viscosity up to ISO viscosity grade (VG) 100, as defined in ISO 3448. Within the range described, the filterability as defined is not dependent on the viscosity of the oil. The procedure is not suitable for some hydraulic oils on which specific properties have been conferred by the use of insoluble/partially soluble additives, or by particularly large molecular species.