I haven't seen any numbers/tests, but I can't believe some coarse steel shavings--even soaked in oil--would filter as good as oil-treated cotton or standard paper filter material. However, I suspect the stock filters actually do a decent job, certainly better than nothing, and probably don't impede airflow very much.
Yes, the stock filters should be oiled. When I ran them, I used a special 'tack' oil formulated for this purpose (from Amsoil, IIRC). It seemed like the oil mostly ended up in a puddle in the bottom of the filters anyway.