i know what the book says, but im wondering how many miles i should be able to get out of the factory replacement NGK plugs. i just pulled my plugs to see if i could figure out why im smoking a dark gray color. and the plug looks kinda burnt up. its dark red and splotched under the electrode. what do i need to look for to know if its needs changing, its not oily at all. the electrode is gray. i coulda swore i just put these plugs in about 30,000 ago. maybe i should just change the bitches anyway. but could that make me smoke?

 

You put any octane booster in (red discoloration)