Hey Eric,



The stock paper air cleaner does a fine job of filtering the air. The paper filters actually tend to filter better than the K&Ns do. Any performance or economy gain would be minimal at best. One thing, its very important to oil those filters sparingly after cleaning since over oiling them can cause the oil to muck up the MAF sensor. I think the biggest advantage is the K&N is able to be cleaned and reused. You can read up about these and other filters on the bobistheoilguy forums.

http://www.bobistheoilguy.com/

Lots of interesting info about oil, air and oil filters and other related topics.

I would just keep a clean Mann filter in the box and make sure your airbox thermostat is working or disabled.

Cheers, Jon 'Uncle Olaf'

Thank you, all! Think I'll save my money for more IPD goodies instead.

Without some kind of heat shield to separate the filter from the rest of the engine compartment your probably going to see a decrease in performance, the proximity to the turbo and high underhood temps will be the reason.
I swapped mine not for any of the standard reason but because my stock airbox was broken and would not seal correctly. I did fabricate a heat shield and make allowances for outside air to flow to the space occupied by the filter. even with that type of install there was no noticable difference in performance.

I dropped a K&N OEM replacement in my 98 S90 about nine months ago. I keep pretty accurate gas mileage records on the car, and I saw a small improvement in gas mileage. Let me follow that with this, at the same time I also swapped from dino motor oil to Mobil 1. The increase in mileage could have been the oil and not the filter. As for improvement in performance, I can’t say I saw any.
All that been said, I am happy with my K&N filter. The main reason I swapped was not for performance, gas mileage or longevity. I swapped to the K&N because “I” feel that it is a better filter overall. Filtration been the main reason for a filter, I think it provides my car with best solution for clean air. Now, the fact that I don’t have to buy another filter and just keep this one oiled and clean, well that it is just a bonus.
Bottom line, it is a personal preference. I don’t think I will break even on the price of the filter versus the cost of replacing paper filters, but maybe it may help my engine last longer. Maybe? Good luck on your decision.