Air filters. If they are the same size as the factory and they flow more air, then they will flow more dirt or air borne particles. Just physics.

Just an observation. I have both a 88 Twin cam air filter set up, including stock base plate, filter, cover, breathers etc and a stock Evo set up with backing plate, filter, cover, breathers etc. When you compare the two, the stock Twin cam set up to the Evo, the EVO setup has a bigger filter ( about 1/4" inch thicker, ie: more filter material ) otherwise the same overall dimensions as the Twin cam filter. Also the EVO air filter cover has larger openings. The head breather bolts on the EVO have a larger inside diameter than the Twin cam breather bolts. So basically the stock EVO setup potentially flows more air than the stock Twin cam, however the Twin cam air filter cover is bigger so there may be just as much air available for the engine. What's called dead air space is important, so somebody with a air flow bench of some kind could likely see if there is really much difference between the two.

I guess when the 88 Twin cam came out was when HD started to really restrict the intakes and exhaust on these bikes. I personally am not a fan of the oiled filters, as it just ends up coating the intake. In an ideal world, if you could stack two air filters together ( like the old car racers use to do ), you would have the best of both worlds.