which oil

[twin200]

I was talking to my diesel guy today about oil for the 6.7. We both have always run 15w40 rotella in our 7.3's. He told me ford says 5w40 for the 6.7 and that the computer is using this viscocity to make calulations for oil change intervals. I almost forgot(synthetic). Does anyone know different

[CascadeF250]

The manual says you can use 15/40 at temperatures above plus 20 degrees. Below that is recommends you use 5/40 weight. I also would like to run Delo 400 15/40 but am not sure if I should. I have not changed oil yet and have 4400 miles on the truck. I would like to run 15/40 all year long and not have to worry about going to a lighter oil in winter. The manual recommends 5/40 for severe duty.