I had one of the very early 6.0's (an '04 with a late '03 engine), when I got rid of it I don't remember it having a single original injector left. On cold winter starts, 10w30 redline syn did OK, but the engine still ran rough for a minute or two-15w40 was out of the question. A few years back we had a record cold snap, -14f and that wasn't wind chill (i had frost inside the windows even after two hours of highway driving with the heat going full blast). The truck barely loped to life, and nearly stalled two or three times in the first five minutes of driving. I was parked away from home and had no place to plug in the block heater. I was driving from U of I champaign to NW Indiana, and got as far as I74 before things smoothed out. 5w40 in just about any flavor/brand solved the issue. I went one better and solved the problem with a cummins, but that's a story for another thread.