06 6.0 Powerstroke, 134k, all stock. Only mod is coolant filter. I run Archoil and a fuel conditioner. I recently switched from Diesel Kleen to Optilube summer, which is supposed to provide better lubricity. Was getting 15 mpg mixed highway/around town driving. I'm now getting 13. Truck runs excellent, otherwise. I see some black smoke when under heavy acceleration from a stop, but never any other time. I have a Scangauge II. No codes, thus far. Any suggestions?