Does anyone here understand why some oil filter manufactures report their efficiency rating as greater than some micron size, and other companies will report the efficiency rating as AT or @ some micron size? For example, the STP S6607 oil filter has an efficiency rating of 93 percent for particles size greater than 20 micron. What does that mean? When STP introduces contaminants into the oil during oil filter efficiency testing, what size are the contaminants? I guess what I am asking is how does STP arrive at 20 microns? When an oil filter has an efficiency rating of 95 percent @ 20 microns, does that mean the contaminants used to test the oil filter efficiency rating are all 20 microns in size? All the filter manufacturers that report their filter efficiency rating differently say they test using ISO 4548-12 standards.