The backside of filters never appears dirty cuz the dirt doesn't hit that side..

I and many others on my taco board have found a fine layer of dust all along the intake tube after the filter.. Some don't see it, some do, but it just wasn't worth taking any chances anymore so I just gave on it.

The real kicker is when you hold the K&N up to the light, you can actually see some of the holes in it which are only about 100x the size of dust particles. The oil traps whatever dust hits it, but if the dust don't hit the oil then it goes through the larger holes instead.

K&N filters are made so that they give you every last drop of HP you can get, these were made for the racing industry who rebuilt thier engines after every run. On the other hand, I'm an offroader and I run in dusty conditions and I care more about teh filtration than 3 extra hp. Now, dont' get me wrong, running a K&N wont' destry your engine, it will just be noted that at 100,000 miles you won't have as much compression that you would have had and so on.

I now run an amsoil foam filter which is about 2" thick. It's an oil type filter too, but it catches a lot more of the dirt and flows almost if not just as well as my K&N w/o letting in the harmfull dust.
-Chris