While trying to avoid overstating the obvious, consider this: Micron ratings are important, as mentioned multiple times in this thread, and they are not to be ignored. But, three other vital factors include: Quality and consistancy of the bypass spring; Durability and fiber content of the filtering element; Total surface area of the filtering element.
I purchased a filter cutting tool from a Caterpillar dealer in 1984. Routine use of this tool, combined with a clean utility knife, a clean shop towel, a vise and 30X jewelers loupe will reveal everything you have been missing.
1. Place used filter in vise, gasket up
2. Use filter cutting tool to remove top, gasket end
3. Remove filter from vise and invert, separating the filtering element and spring from the canister (Bypass spring resides against end of canister opposite of gasket) Many, "Economy" filters use no spring at all, substituting a non-forgiving piece of stamped steel.
4. Using clean utility knife, cut and remove a 3"x3" or larger section of the element
5. Wrap a clean shop towel around the compressed folds of the element
6. Place folded towel and element in vise
7. Tighten vise to remove excess oil from element (Shop towel absorbs excess oil)
8. Unfold element
9. Insert jewelers loupe (In GOOD eye)
10. Fan out and examine surface area of filtering element and the various particles trapped therein
< Many, many particles too numerous to mention in a high quality filter > Passing a magnet over these particles will separate the tin overlay, steel, etc., from other materials left behind.
Enjoy,
Charlie