Yes, I'm Canadian, but I don't plan winter driving.
I suspect that I'm OK by lowering the lower end of the weight from 15W to 5 W or 0W, since that only affects the engine when cold, and it might help oil flow when starting up.
It is important to keep the upper end at 40, since that is what the engine does when hot.
But, I'd really like an authoritative reference from Cummins to confirm, since it would affect my warranty.
The do suggest not using synthetic until after the first oil change.
Gordon Sick, Calgary
2015 Berkshire 34QS
2005 Acura EL (aka Honda Civic)