The correct way to measure how good a filter is through the Filter's Beta Rating and the fluid contamination level according to ISO 4406 which is expressed as a 3 digit number such as 18/15/12. Beta rating is essentially how efficient the filter is as removing contaminants of a certain size. There is no such thing as absolute filtration and you would need to have a beta rating of ∞. Example for bulk filter is Donaldson P568666 which is 4 micron @beta 2000 and targets a cleanliness level of 14/13/11. What this means the filter captures 99.95% of particles 4 micron and larger (Filter beta Rating) and for a single pass through the filter you can expect between 8,000 and 16,000 particles 4 Micron or larger, between 4,000 and 8,000 particles 6 micron or larger and between 1,000-2,000 14 micron or larger in 100ml of fluid. Most filter companies do not publish this information to the consumer but it is available in the engineering documents.