I have been running Mobil 1 Full Synthetic 0w-40 in the V8 4R, I believe, since I got it in '13. No issues, great used oil analysis. The 5.0 (using regular Mobil 1 5w-30) is coming to the end of its warranty and am thinking about using it in this V8, too, as it's a great oil and want to simplify the oil usage/changes/top-offs. I know there are a few guys using it on mustang6g.com...haven't heard any issues. Just curious. 