I run System 1's 30 micron stainless steel mesh filter in my car... PC Racing's FLO 35 micron mesh is ideal for heavier weight motorcycle oils.
The drop-in type is pretty basic and looks like an OE oil filter cartridge but with the stainless steel mesh as the filter media, rather than the typical filtering media. PC Racing claims that the filter has 200% more filter area than a typical filter, along with a flow rate up to 7 times higher.
The surgical grade (claimed) 304 stainless steel mesh filter media in the Flo Oil Filters is rated to filter down to 35 micron size particles. One micron is a millionth of a meter, or 1/25,400 of an inch. 30 microns is about the size of maybe your typical speck of dust or a mold spore. A grain of sand is about 100 to 2,000 microns.
The key point here is that the stainless steel media filters down to this size consistently across the entire surface area. Mass-market motorcycle filters are mostly unrated, because of the way they're constructed, with all sorts of holes and passageways through the media that are not consistently sized or shaped at all.
Now I have seen synthetic media (non-automotive) filters that filter down to a claimed 0.8 microns in size, but you also have to consider the pressures and viscosity of motorcycle oil. I doubt very much that an 0.8 micron (or anything close) filter will hold when a motorcycle full of 20W50 is started up on a 40-degree morning!
UPDATE: Many traditional filter manufacturers either make no claims for the particle filtration size in microns. Also, there are "single pass" claims (filtering oil with one pass through the media) and "multiple pass" claims (more than one pass through the media). We checked the K&N website (no claims); Wix filters (claim 25 microns) and Fram (20 microns with multiple pass). Other sources say that any manufacturer's claims may be suspect.
Also, there are several factors to consider, including the filter capacity, the total amount of contaminants trapped between oil changes, oil pressure differentials at varying speeds and temperatures and more. So oil filter performance seems to be based on more than just a claimed particle filtration size.