What they don't tell you is what is the biggest particle size it will pass. Fuel filters have about an 8 micron rating with a 25 micron max. The tradeoff here is flow rate. The higher the filter efficiency, the lower the flow. Engines are pretty tolerant of particles up to about 30-35 microns so the small end of the scale is really not what you worry about.
Most filters actually improve in efficiency with use.
That is because the big holes get bridged over and the surface becomes more uniform. The best quality filters have a micron rating that is fairly high, say around 28 but is very uniform pore size. This paper improves pretty fast to around an 18 actual perfomance and stays that way a long time. The small pores tend to plug over in use and the big holes open up with the added flow through them. This results in a worsening efficiency condition.