Are you saying within a few seconds/minutes it goes away? I think the roughness on start up is a result of the calibration. Start-up fueling and ignition timing is a bit of a black art that they calibration engineers more than likely figured out by trial-and-error. It will be specific to an engine, and I don't think any oil choice will fix it. It is more likely that your engine/transmission mounts are worn, allowing more engine movement or more transfer of vibration to the cab.