For discussion's sake:
Diesel engines produce more carbon, soot than gasoline engines by their design. Their oils too are different since diesel oils have to contain the abrasive carbon til it reaches the filter- thus more detergents in diesel oils.
So why would a new nissan engine (with common rail, EGR) specify a lower API rating to the point of using oils primarily designed for gas engines? The TBN in gas oils are much lower, therefore easily depleted when used with high sulfur diesel fuels. The lack of TBN would lead to higher acidity in the oil which could lead to a shorter engine life.
On oil prices:
It's possible that Shell Helix diesel was first designed as a gasoline oil then broadly priced as a diesel too- but only spec'd at CF. The Rimula line was meant for diesels and priced as one.