In diesel engine studies (SAE 864507), the wear rate for particles less than 10 microns was found to be directly proportional to their weight. What this means is that for practical purposes you cannot filter particles too small. There are more particles the smaller the size, and all sizes are bad.By filtering down to 1 micron efficently you remove around 3/4 particles by weight, hence you decrease wear by 400%.This is the rule of thumb theory .A particle oil count before and after installing a bypass filter should show the reduction in particles, and the reduction by particle weight not particle numbers would be directly related to increased component life. How anybody arrives at a critical particle size is a matter of determining your current engine lifespan with your current particle count, and then projecting how lowering your particle counts will improve your wear rates.