Depends on how cold it gets 20 below for sure.
Diesels for sure and have to use #1 diesel fuel #2 fuel gels up.
Had a lot of truck drivers from down south come and work in the oil field in the winter lots of semis on the side of the road from gelling up and it is hard to thaw out the truck.
My son has that new 6 cyl. diesel in his ram getting 30 MPG. Really nice truck.