They use ASTM testing so their micron rating is not a Beta or average of multiple passes like most brands advertise. 35 micron is a single pressurized pass through the element. IMO the ASTM way of testing is what everyone should advertise. I'm not an oil engineer but the other ways of getting averages to make your rating look better seems like brands just trying to pull a fast one.
From K&P
"We use ASTMF316 testing procedures which eliminate many of the user variables found in the SAE procedures. Basically, the filter media is pressurized from one side, and when the media starts passing particles, that is the micron rating. We sent filter media from several common brands of paper filters to the lab to be run through the ASTM test. We sent the media to the lab with no names, just numbers for identification so they wouldn’t have any idea what brand filter they were testing. The results for the paper filters ranged from 48 microns for the best filter to over 300 microns for the worst filter. Our tests were right in line with other testing results we have researched that have paper media filters passing particles anywhere between 50 and 90 microns. What does this mean? Paper filters are rated on averages, percentages of efficiency (also known as beta ratios) and multiple passes, so a 10 micron rated paper filter (as advertised on the packaging) may be letting particles 50 microns and larger through. The medical grade stainless steel cloth that we use is consistent across the entire media surface and is rated at 35 microns, meaning nothing larger than 35 microns should pass through the material. The bottom line is we meet or exceed the filtration performance of OEM filters, eliminating any warranty issues."
