Many will disagree, but here goes...... Be carefull with the 2-letter air filters mentioned previously. They will improve seat of the pants feel a bit, but they can cause other issues. They are made for high airflow, this means BIGGER holes. These bigger holes allow more air while stopping low flying birds, sticks and rocks. The oil added to the air filter will catch the dirt and dust. However, the oil(with dirt, mixture) can and will be sucked into the intake becoming a valve lapping compound(10x worse on a diesel-leading to a dusted engine). On a gas burner, if the oil is not added in the proper ammount you will either be too dry letting in more dirt. Or in the other extreme, you will have too much, this will cause the MAF sensor to become contaminated causing a lean condition because the PCM doesnt know how much air is actually being ingested by the engine.
With that said, these filters have their place. It is a much better choice than no filter at all. If you are looking for more performance and dont mind the idea of premature overhaul then it makes alot of sence. If on the other hand you would like your investment to live as long as possible then you are better with a factory air filter. The factory filters are typically better than the average aftermarket because they are product specific and dont fall into the air box as it wears out.
OK, way off topic...... Just wanted to throw that info out there, if you disagree, thats fine. Please do some independant research on these filters before buying on.