[Hammer]

I have heard of the filter oil in K&Ns causing problems with MAFs before, but it was almost always because of the user over-oiling after "recharging" the filter.
As far as the contaminants are concerned, If you have one of the standard cold air kits that extend into the wheel well, you are definately exposed to more dirt, rain and god knows what else. Although the filters are designed for such things, I would try putting a K&N in the stock air box without the air silencer and see if that helps your dirt problems.

As a side note, I had my K&N filter on my 98 GT for over 60k miles and never had an issue....