I lost a bit of faith in oil analysis recently. On an airplane I maintain I cut open the filter and pulled the screen...they were both full of small flakes of trash (aluminum), maybe .005" or so in size. Out of curiosity we sent the oil off to Blackstone to get analyzed just to see what would show up. It came back good. In the mean time a bit of investigating revealed the magneto drive gear bearing which is only used on the IO-360's and 540's seized and trashed the bore where the it fits into the case, and of course the bearing disintegrated. I pulled a cylinder which was running low on compression and when I did...the rings fell on the floor in pieces. Looked inside the case and saw lots of evidence of pieces floating around the crankcase... needless to say I'm overhauling the engine now.
When I inspect filter pleats I normally take the filter media out of the can, cut it into two pieces, fold them back up flat, and put it in a vise to squeeze the oil out. Works like a champ, and if I didn't do that in this case, I don't think I would have seen the trash in the filter. You can't see trash in the filter if there's oil covering it up. In this case if we went off of the oil analysis and did a cursory inspection of the oil filter its very possible that something bad could have happened.