10w30 is what Ford now recommends for the Powerstrokes. The 7.3s and 6.0s do way better with it being that they use the oil to fire the injectors.
It also stays in grade better than 40 weights in them. The fuel system shears the oil pretty bad in the 6.0s. If you start out with 15W40 it will be closer to a 30 weight on the high end then a 40 weight as a result of shearing. The 10W30 will stay a 10W30 and it also helps stop injector stiction.
Actually, 15w-40 oils are less prone to shearing than 10w-30. In order to get the SAE 10 or SAE 15 base oil stocks to their respective high temperature weights of SAE 30 and SAE 40 respectively, the viscosity must be increased by 3 in the 10w-30 and 2.6 repeating for the 15w-40.
In order to do this, VII additives, which are polymer chains, are added. These polymer chains are what generally shears in oil, and there are more in 10w-30 oils than in 15w-40.
Maybe there's a different reason they've gone to 10w-30, but shearing doesn't support it.