A 5w-40 diesel oil is what I would use.
Enough Zddp for that older 5.0. Delvac, Rotella, ect. Readily available, and reasonably priced. Is that 5.0 a Ford engine?? Ford in 1989, or there abouts, changed the engines in Mustangs to roller lifters. Perhaps you have one of those blocks in a marine...