This part of ISO 13357 specifies a procedure for the evaluation of the filterability of lubricating oils, particularly those designed for hydraulic applications, in the presence of water. The procedure only applies to mineral-based oils, since fluids manufactured from other materials (e.g. fire-resistant fluids) may 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. NOTE Filterability is a prime requirement for lubricating oils used in hydraulic systems because of the fine filters used in this application. This part of ISO 13357 defines a method for assessing the filterability of oils in the presence of contaminating water. It should be noted that some oils will exhibit poorer filterability characteristics in these conditions. ISO 13357-2 should be used to investigate the filterability of an oil which is to be used in applications where the presence of water in the oil is unlikely. An oil which has good filterability in the presence of contaminating water will not necessarily have equally good filterability in dry conditions. An oil having good filterability only when wet is unlikely to be generally acceptable.