In 265K miles I've never used an additive in my diesel truck. The only exception is if I'm going from a warm area to a cold area, then I'll use an anti-gel. Example, being in North Carolina in February then driving back to Minnesota.
Where I have spent extra money is on more frequent fuel filter changes. Manufacturer suggests changing fuel filters every 15,000 miles. I have always changed mine every 7k to 8k miles. At 218,000 miles my injectors were removed, inspected and new O-rings installed. They went right back in the truck.
