It wasn't really a problem for me. The only reason I even bought the bottles was for another car that idles like shit without it. I figured, why not try it, see if I noticed anything. Bear in mind when I did this, I was going through 2 or 3 tanks a week. So any changes that my car had I would notice pretty quickly. It's something you'll notice that you were missing but had long forgotten about pretty suddenly coming back.