would it be alright to add a KN filter
 

I have 08 highlander,when I got ready to change it,I found a pre filter with a air filter.Most people tell me that toyota have a good filter,but I want to save on MPG.The pre filter,is made from nylon bands,I couldn't see very much light thru it.What's your opion.THX Again

K&N
 

My opinion is,don't.Science says you'll never see a benefit at the gas pump.I used to use a Filtron-sock(pre-cursor to K&N),air cleaner on my race bikes when I was younger and more ignorant.Now I'm older and a little less ignorant,and while K&N's hype does speak to my intuitive side,the science does not bare out their claims,and I will use nothing but a micronic paper filter element as is standard on your Highlander.K&N has found a great way to sell cheap oil for alot of money.

I would replace it with a quality air filter like Baldwin and leave it at that.

Autospeed.com did a nice set of articles on negative boost in the intake. If I remember right the air filter was responsible for 7% of the restriction of the whole intake. Even if a K&N would halve this restriction, the impact on fuel economy of an hypermiled vehicle would be lost in the tank to tank mpg variability noise.

fe wise no (toyota 3 stage filter work great as filters. go amsoil filter if u want a better on than k'n)
hp wise fill up with 93 for 3 tanks, and u will feel a difference, that vvt-i and 3.5 (2gr loves high test, but for some reason for toyota to get the full effect of anything about 3 tanks worth must be run through it.