I have a 1991 Grumman Olsen box truck with a Ford 7.3L diesel. I don't really know how many miles it has because it only has a 5 digit odometer. But I assume its a lot because the truck used to belong to the newspaper distribution and it got a lot of use. I know that the oil in it right now is a non-diesel 15W-40. I wonder what weight I should use during the Salt Lake City winter, and which one for summer. Thanks

You could use a syn. oil 5w-40 year round. Rotella T syn 5w-40 can find at wally world.

15W-40 or 5W-40 syns are both fine all year round. Engine might enjoy the 5W in the winter a little better:)