I run the K&N in my 5.0l Mustang GT. There is a 'K&N' air filter setting on some of the tuners. That tells me, they are letting more air into the engine. Re-mapping with the tuner takes advantage of it. The OEM paper original seemed very restrictive(I posted a photo). I am getting almost 19mpg city. My mileage went up about .5mi a gallon, with the tune and air filter change. 1/2 to wide open throttle feels better. My tires break loose stomping on it at 15-20mph. So yes, maybe 3-5 cheap hp and maybe a little extra mileage. My Expedition and Lincoln have the Fram AIRHOG. They look like they were made at the K&N factory, but orange. I found them on clearance for $20. I am getting around 18.5mpg with the 32V V8 Lincoln and it runs awesome. The Expedition runs good, but mileage sucks(12mpg). The OEM paper filter, was a unique round design with a plastic nose cone. I have never seen one like it. My guess it was probably doing a fairly good job already