So much for an experiment with only one variable. Yesterday i changed my oil, I've always used rotella before but i switched to mobil delvac. I also added (for the first time) Opti-Lube Oil Fortifier. And i installed Vulcan Diesel Performance's 2mc fuel filter system and big line kit (1/2"). I don't know what made the difference but today my truck runs smoother at idle, idles quieter, runs smoother when you mash hard on the skinny pedal, and has a little more go. I don't know what made the most difference but i love the combination. Would highly recommend all of these products. P. S. I have also been running Opti-Lube XPD for about the past month now and i am very pleased with it.