When they changed the spec they called out MS-10902 or Rotella T6 5W-40, only concern I would have for 15W-40 don't use it below 40*F ambient temp.
I've run 15W-40 in my Ecodiesel in the middle of summer, no concern of cold starting. They specify 5W-40 it can be used year around in US Climate, easy decision.