I once switched to 5-40W Shell Rotella full syn and when I adjusted my valves the engine head was really clean, but then I started to see more blow-by. So I switch to Chevron Delo 400 15-40W full syn and that's what I run now in both my generator and engine. I don't recommend 5-40W for warm weather driving. The 15-40W is best for spring -summer-fall driving above 40F.
2004 Itasca Horizon ISK-40AD, 350HP Cummins 8.3L
115K miles and the best of 3 Diesels I have owned thus far!