I have a 2012 6.7 HO Cummins. It only happened to me once. On my fourth oil change @ 17500 miles, I checked it when I got home, a whopping 6 miles away. It was every bit as black as when I took it to the dealer to have it changed. I thought the dealer changed the filter and topped it off with fresh oil, and still do. Everyone here thought I was a total moron. Before that change it never happened before and it hasn't happened since. Last oil change, 76400 miles. It has consistently stayed somewhat clear for about 40 miles after the change. I don't think it's normal, but hasn't seemed to have hurt anything either. About 30000 miles ago it got a clean bill of health from Blackstone Lab and runs fine.