That depends on the oil and how its going to sheer as well as the engine it's going in. For my 5.7 with its 180 T-Stat it really likes the ~12 CST of Redline 5W30 or PUP 0W40 once it sheers down. The oil does run a bit colder at around 195 empty so it's going to be a touch thicker than the advertised hot visc at 212. The 6.4 being an SRT has higher compression (10.9:1) and gets hot once it's been worked so the ~15.5 CST of the RL 0W40/5W40 works well there since the oil is usually right around 210 with a 180 T-Stat and can get up to 240-250ish when it's being pushed for a while. Since the 6.4 truck configuration has that 216 T-stat I would probably shoot for around the same, taking into account how much the oil will sheer during the OCI. If it's under high loads then the thicker rated oil, again in that 15 range, will thin out a bit at elevated oil temps and maintain better protection.