Really, when you get down to it, most ANY high mileage, full synthetic oil is going to work to your benefit. The fact is that the seals and gaskets on a 27 year old vehicle are going to show signs of hardening and stiffening, from thousands of heating and cooling cycles over the last quarter century plus of operation.
Leaks are going to happen. Much like grey hair on a Social Security recipient. It's the nature of the beast. By using a high mileage, full synthetic oil, it will have a good additive package to help prevent this. And the more you can put off having leaks and such, the better.
I've even read where some guys use high mileage oil right off the bat in a new vehicle. This to help keep seals soft. Supposedly high mileage oils also have good additive packages in them, that make them more desirable. Regardless, at this stage in the game, your truck is taylor made for it.