A micron is 1 millionth of a meter, or .00003937 of an inch. The micron rating of a filter determines the particle size able to pass through it. Different engine manufacturers recommend the micron rating for filters used on their units. Sometimes it's ppm, or particles per million, referring to contaminants of a certain volume. For example, gasoline engines can run on larger particles if their carbureted, where injectors will not accept them. The viscosity of liquids being filtered also determine micron ratings. Luckily the filter manufacturers did all the engineering for us and know what to supply for each system. So I try to always buy a good name brand filter. Unfortunately small engine filters are usually just a screen with unknown mesh size and we use them because their small and convenient. I recently started buying leaded higher octane fuel for my motorbicycles and immediately they run better and idle smoother. My hoses stay more flexible, it doesn't ruin the paint on my tanks, they start easier and get better mileage. I use sediment bowls on some of my bikes with good results, but I also like a valve with an internal screen to keep from sending debris from the bottom of the tank to my engine. Hope this all makes sense.