I bought a TJ a few weeks ago after driving Cherokees for nearly 20 years. A colleague is a gear head and has a 2000 TJ, so he immediately came over to look mine over. When we popped the hood, he pointed out that gas mileage and performance would improve with an aftermarket breather system like a K&N. I was looking at the K&N on a few online sources, as I was familiar with them from my road bike days. However another Jeep friend told me that an offroad magazine rated K&N pretty low and others superior. Opinions from those who have used specific models/brands would be very welcome.