I have a 2008 STS V6 that I have owned about three months. I read all the hype about K&N air filters, so I thought what the heck I'll try one. I don't know about increased HP but my highway mileage went from 25 MPG to 27 MPG - same 400 mile trip, same speeds etc. I am now a true believer. Best $50 I ever spent!. BTW a truly great car; my other one is a 92 STS.

Did you buy a paper filter replacement (drop-in) or a cold air intake (cone)? I've been considering the cold air intake. Had the drop-in filters in the past with mixed results over time.

for my Caddies, I use the K&N that fit where the standard filter used to be, but the Allante is already a cold intake filter ..
for the Stang, I did mod the intake to make it cold intake .. the cone.
both the square and the cone are built the same way .. as far as i can tell

i also do the unthinkable .. soak them in solvent now and then .. then spray them with WD40.
NO, i havent had any more carbon build up in the trhootle intakes thatn i had before.

for the 4x4, i stay with the old paper fitler .. too much dust, sand, etc ...

It was a drop in filter. I am very happy with it.

2+ mpg. Thats good to hear. I've put K&Ns in all my cars, and its always made an improvement in performance and economy.