I run 89 per the manual. I tried every combination... 89 and 91 with stock tune, then 87, 89 and 91 with the associated superchips tune; 89 w/stocktune was always the best mpg and no significant performance increases with the higher tunes. Though i'm relatively easy on the gas so didn't really put the WOT tuning to the test.

I usually clean my K&N once a year. Take a peak at it... if it's not dirty then don't worry about it.