Jerry Ramey, I'm not exactly sure what is required, test equipment wise that is, to determine what a given filter's micron rating is but some filters say right on the box what their Micron Rating is; For Example, Mobil 1's filter box states 10 - 20 Micron Rating, Fram doesn't state on the box but their tech. bulletins state from 15 Micron's (H.P. or High Performance Filter )up to 25 - 40 Microns on their high-volume filters sold thru volume retailers. Amsoil's Specs. show them at 4 - 6 Microns; they are absolute at 20 microns (one pass thru the filter gets 100% of the contaminents at 20 microns and greater). The numbers for any given filter should be pretty easy to obtain. For those of you not so familiar with wear characteristics directly related to micron size, the difference between 4 microns and 20 microns is huge ! Another positive note for those of you using Synthetics, you can filter them much tighter because of the flow / pumpability characteristics and thus keep your oil much, much cleaner for much longer intervals. Are there filters on the market, other than Amsoil, that are designed for synthetic oil Only ? That's right, Amsoil's filters are not designed to be used with Petroleum Oils. NOTE: All of the numbers provided above are for full-flow filters ONLY; the by-pass numbers are much tighter than even 1 micron.