Oil filterability measure iso 13357

ISO 13357-1 (wet oils) has two measurements in two stages to determine the estimated performance of the fluid in service with regards to its interaction with the filtration systems and water (2000ppm).
Stage 1 – measures the mean flow rate through a membrane filter, i.e. the best scenario when the filter is not blinded at all.
Stage 2 – measures the difference between the initial flow rate and the rate at the end of the test, i.e. measuring the effect the oil going through the filter has on flow rate.
Stage 1 is generally the main part people look at as that is the main in service issue, but stage 2 is designed for more extreme scenarios and where very fine filters (e.g. <5 micron) are used as stage 2 is particularly sensitive to gels, silts and oxidation bi-products.  
The limits used typically are designed for ISO VG 100 or less.
The pass value is >50 and <50 is fail, in really bad samples if takes >7200s (i.e. 2 hours) then
ISO 13357-2 (dry oils) is a less strict environment where water contamination is not likely and water is not added to the sample.
Both methods have a problem in diagnosing correctly if the sample submitted has water contamination as then it is not dry and then adding water to the sample makes it worse.