I hafta 2nd Jeff's explanation and add that part of the action that happens is the air velocity throws up a fine film of oil onto the mesh which constantly 'washes down' any dirt that makes it that far - read yer shop manuals, there's a wealth of wisdom there!!
the oil bath filters were/are exceptionally effective and were first made 'extreme service' options, then dropped, because it was cheaper to make the dry element filters, and because lazy owners and mechanics decided they were messy and not worth the fuss - they're the absolute best type to have in dusty conditions and at least as good as the dry elements in normal use
Bill
