I have a K&N CAI. You won't get power that you will realize. Check there web site, even it says something like 7-9 hp. Strangely enough, I use both the K&N oil/gauze filter and an Amsoil dry filter. Better air flow means less restriction and thus lower engine pumping losses so more HP and MPG. This is usually at the expense of filtration unless the filter also has greater surface area, as with a cone filter vs stock panel type like the ones in our Explorers.
Oiled filters have the advantage of being reusable. That is, they get dirty just like any other filter but you clean them and re-oil them. Near good as new. You don't just drop them in and they're good for a million miles. They also have big holes for the air to go through so they offer much less restriction= more power. Good for racing but not so much for long term street use. Another down side, If you use too much oil (easy to do when cleaning and re-oiling them) you risk contaminating the MAF sensor on a fuel injected engine.
So why do I use one? I use the oiled filter, K&N, when off road on a very dusty surface. Why? I trade the slightly worse filtration for the ability to clean it on the go and drop it back in. Much cheaper then replacing a paper filter even more often. As for the Amsoil, best of both worlds. It too is cleanable/reusable. I just find it needs cleaning more frequently- it seems to trap stuff more on the surface than down inside like an oil filter and oil works pretty good at attracting small light pieces of earth.
Unless the small increase in power is worth the large price for a CAI you are left with something that fits the stock air box. That means paper or K&N drop-in oil filter as best I know. If that were my choice, I'd stick with a quality paper filter. If you have the option of a cone filter, by all means do that. It has a much larger surface area. Then I'd use the Amsoil for all but extremly special circumstances.
