0W40 synthetic is an interesting spec.

I guess the "40" (when hot) part is to protect heavily loaded bearings when running hot and hard in a truck application. Same idea some hot rodders have (that includes me!) when running a warmed up engine hard for extended periods.

I guess the 0 part is for fuel economy on cold start. I think this is an EPA test? Also, the MDS system does not like thick oil so to some extent, lighter cold viscosity is necessary for MDS.

I guess the synthetic part has two drivers. One is to get a durable oil with such a wide range of viscosity. Another is to handle extended high heat in the oil. Does the 6.4 have an oil cooler? I am pretty sure the 5.7 does not.

An experiment would be to run some conventional 15W40 in a 6.4. I bet it would hold oil pressure OK in hard use... at least for a while. Also the MDS may not work.

This could be an expensive experiment and I would not run it personally.