I'd wait for fuzzy to chime in he is the diesel guy around here.

My non-diesel opinion would be to put what the manual suggests. It just doesn't get all that cold around here. A little colder where you are then here in the triangle but not much. Nothing like up north/mid-west.

For the Winston area the average low in January is 25f. The average High in July is 87f.

I'm not a fan of full synthetic regardless of engine or climate.