One additive that I swear by is Z Max. It's alittle pricey about 25 bucks when on sale, but over the last 3 years iv'e used it all my cars seem to run smoother then they did . The manufacturers claim the linkite base soaks into metal anyway it seems to work, and even if it doesn't the myth makes me feel better.
92 Pearlie 146k home of the mysterious thunk, thunk. thunk,............................................ still think T converter or flexplate. my nemesis