I also run synthetic 5w 40 year round which is recommended for the cumming 6.7l. Was on the amsoil and shell websites. Could find any field tests or comparisons on the shell site. Info on amsoil. AMSOIL Premium API CJ-4 Synthetic 5W-40 Diesel Oil
In my '06 Dmax, I've been running Case IH 0W40 synthetic, year round. The nice part about it, it comes in a 9.46L jug and it's the perfect amount for the oil and filter change. Since sold it to my pops and it doesn't miss a lick.
used to run 0/30 year round then switched to 0/40 when it came out. nice low temp viscosity , and the viscosity on the top end is pretty much the same....good for all year. also dont have to worry about fall time when you just changed oil and the temp drops way off