I've been torturing myself with this question as well. K&N will send you their test data with no hastle. That data showed a dust filtration efficiency of 96.8% in the fine grade dust test. The test dust was to SAE Air Cleaner Test Code J726, Fine Grade.

From what I understand that efficiency rating is decent. I don't think generic paper air filters would be much better. The real question is what is the efficiency rating of the stock Volvo branded air filter.

On the other hand, there's quite a bit of writings on the BMW boards and many concerns have been expressed. For example: http://www.seansa4page.com/resource/airfilter.html I would expect the stock Volvo air filter to be similarly good.

On the other hand, IPD has had many years experience with the K&N filters and would NOT sell something that they felt would be harmful to your car. They run the K&N in their own cars and can cite examples of having done so for 200K+ miles in the same car.

On the other hand, is the dust in your area similar to the dust in Oregon where IPD is located?

On the other hand, IPD has never had any complain about a K&N damaging their engine.

On the other hand.....:)

So let's do a poll. Who out there has run the most miles with a K&N?

Good luck!