not trying to steal outlnder's thunder BUT..........
if you do some research on here the top 2 CAI's for our Frontiers are Volant & Injen !
i have the Injen CAI (pic in my gallery) with a Gibson S/S exhaust and absolutely LOVE the combo, my truck really purrs @ 40 mph and even better going up the PA turnpike @ 80
most reviews i have read on here about the K&N didn't rate them very well, i had 1 on my '07 charger and was happy with it but i listened to the great folk on here and went with the Injen after some fitment issues with the Volant.
IF you decide to go with a CAI i suggest you read the reviews/posts and just don't go with 1 persons opinion (mine or anyone else's)
BEST OF LUCK IN WHATEVER YOU DECIDE !