Synthetic Oil - Do you use it in Cummins Engine?

The 10w40 synthetic rotella T6 here cost 104.00$ for 5 gallons, the 15W40 Diesel cost 83$. For the difference I would go for synthetic.

But I never tried this engine in very cold weather with 15w40, I know my Ford needs the 0W40 to be sure it will start in the minus 30 temp.