:4: The main difference between stock replacement K & N filters and ordinary filters like Fram is the material that they are made of. Your typical paper filter will over time clog with dirt and swell from moisture restricting the airflow. These types of filters are known as surface loading. Contaminants are captured on the surface and as the surface loads up less air gets thru. The many layers of oil impregnated cotton material that K & N is made from will not clog like paper; it is known as depth loading and actually captures more dirt than stock filter. This type of filter can hold more dirt per square inch without restricting airflow.
Please take the time to read through the impartial tests (especially #2). In the past, I also blindly believed the advertisements and installed several K&N air filters, the first 12 years ago. Any miniscule HP gain was unnoticable, but the grit deposits in the throttle body over time were very obvious.
Revising the entire air cleaner intake structure for smoother airflow and access to cooler air will increase power, but the drawback is a huge increase in intake noise. This intake howl often persuades people into believing a big change has occurred. The reality is that it is only a few percent more and, if it wasn't for all the racket, it would be unlikely you would feel it "in the seat of your pants." This is the primary reason manufacturers do not employ this system on everyday grocery getters. But hey, if you like the sound - go for it!