Bad mileage
 

04,6.0 f350 dually normally get 13.5 mpg. Last two tanks 9 mpg. Starts & runs good. Maybe turns over a few more times than it used to on starts. Some slight white smoke for a few minutes on cold starts then clears up. Ficm? All filters/fluids recently changed. Amsoil oil & filters. Always used amsoil in the past. Any advise?