I don't know if this is any indication or not but I've been running one of those Unifilter foam filters on my 21hp B&S engine for a couple of years and have never seen any dirt on the neck after the filter. I don't know if that's any indication that it's doing it's job. The engine oil doesn't seem to get any dirtier than when using the paper filter nor is the engine using any more oil than it did before. I guess the real test would be to do a yearly compression test.
When the filter is dirty, I just stick it my parts washer of kerosene and the dirt falls right off. Afterwards I had been dunking it in a jar of 50/50 kerosene and ATF then letting it drip off over night then sticking it back on the mower the next day but I've been using that PJ1 foam filter oil here lately.
Here some pics from a thread I posted yesterday:
https://www.lawnmowerforum.com/showthread.php/57180-K-amp-N-filter-for-B-amp-S-engines
The mower with the K&N knock-off, I made a sock out of that foam air cleaner material which adds a second layer of filtration.
This UNIfilter on my 21HP B&S is not much different than the foam filters used in lawnmowers before they started using paper filters. If these foam filters are so bad for engines as people claim, then how come B&S used them for ages? You could clean and reuse those things over and over until the foam started to break down and turn to powder.
My guess is people these days just don't like to get their hands dirtier anymore than they have to. It's more convenient to pay $15 to $20 for a paper filter 2 or 3 times a year than it is to wash their hands.
The procedure for cleaning the old foam filters was to wash it in hot soapy water (or kerosene), let it dry, then pour motor oil onto it and squeeze out the excess.
Just like they stop putting back-up recoil starters on riding mowers. People would rather just push a button. I guess eventually I'll get to the point to where I can't start a lawnmower due to age by pulling on it and will have to depend on an electric starter.