With ULSD and a 1989 naturally aspirated diesel engine,
a) Is a fuel additive required for “lubricity”?
b) any brand better than any other ?
c) does biodiesel (B5) add enough lubricity to eliminate the need for a fuel additive ?
Sulfur never lubricated anything. That’s “Urban Legend” hogwash. Your diesel engine is just as well lubricated as it ever was. If you want to “jack up” that tired old engine, you can try a little outboard oil in the fuel, which will give a little ring seal and won’t hurt anything…
Personally I would not worry. If you would, then find a brand name additive and use according to the instructions. B5- will add some lubricity, but it also can cause problems with old seals etc.
Overall, I believe there is little to worry about. BTW when ULSD first came out there were plenty of people saying that every diesel engine would be junk by now. Guess what… there has not be a run of problems.