When I had my '97 Tahoe, I was driving an average of 300 miles per day for work. I filled up every day, and I calculated MPG and wrote it on every gas receipt. I did this for about 50,000 miles. One day I mentioned to a customer that I'd love to get better mileage. He recommended the K&N filters. I thought there was no way it could affect mileage with all the computer controls on the engine, but I tried it.
The next time I filled up, my mileage had increased by 1.5 MPG. My average mileage remained 1.5 MPG higher (going from 18.0 to 19.5) until I sold the vehicle with 340,000 miles on it. I did not see any change in oil consumption. I live in very dusty west Texas and was concerned about getting dust in the engine, but apparently that didn't happen. I washed and oiled the filter about every 50K, as I remember. A quick calculation shows a savings of over 1000 gallons of gas. That filter paid for itself many times over!
I tried a K&N filter on my 1995 Suburban, and sadly, it made no mileage difference at all.