Synthetic oil in diesel engines: is it worth it?
Worth it if you start the truck below about 10°F, tow hard, or want to stretch drain intervals. A synthetic 5W-40 flows at temperatures where a conventional 15W-40 needs a block heater, and the makers back longer drains only on their synthetics. For a warm-climate diesel changed every 5,000 miles, a conventional CK-4 15W-40 does the same job for less.