After looking at this thread a second time, I see that "under the microscope there is a hard particle"?
That would mean to me that there is hot exhaust getting into the fuel system under high pressure. I would suspect a cracked head. Somehow the fuel is getting burned to create a hard particle.
Maybe a compression check would verify it? Maybe not. Is there a fuel passage way in the head?
If the filter changes/mileage remains the same, then I would just change them every so often and keep on driving.
JMHO