40u is 0.0016 inch. Most modern day journal bearings are in the 0.001 to 0.002 inch range, depending on the diameter of the journal. When journal bearings are rotating, the actual MOFT (the running clearance) could be down to 5-10 microns depending on speed, load and the oil viscosity. All the wear studies basically show that it's the particles less than 20u that do the most wear because they are small enough to get between moving parts. As mentioned earlier, it's also possible that larger particles can get broken down in to smaller multiple particles, so they can also become a source of the smaller wear particles.
