another oil Question

[rpmonty]

i own a 2015 2500 Laramie with a 6.7 and was told by my mechanic that i have to use synthetic diesel oil because of the turbo. He said regular 15-40 or semi-synthetic was putting the turbo at risk. Is there any truth to this?

 

[crash68]

per the owners manual, 5W-40 synthetic oil is specified when the ambient is below 0°F
No mention of synthetic for use with 15W-40, but it probably better for any engine. There is probably quite a few million miles logged on the CTD with conventional dino in the crank.
Doing a turbo cool down when working the truck is probably more important than oil type.