A 15W30 is a very logical oil, especially for a diesel.
It will contain very little VII so piston cleanliness will be extraordinarily good for a diesel. Compared to the usual 15W40, it will have a lower Noack volatility (a good thing), contain more heavy base oil (also good), possibly contain less DI (surprisingly this could also be a good thing), it will hardly shear and of course give fuel economy benefits.
As it will contain hardly any VII and should contain less DI, it would be a cheap oil for blenders to produce. How much of that saving would get passed onto the customer is anyone's guess but it should be there.