There are no side effects on running the 6's ... If you were to lets say run 7's then you could have a problem with them fouling if you do lots of stop and go .. The one step colder plug will help prevent predetonation under hard use even on a stock engine ... And as we all know , all it takes is one hit of pinging and the ECU drops the timing 3 degrees , which kills performance ... I have always ran the 6's in my Altima .. I am trying to find 7's for the new engine ....