ok, I just had a look, the term "Sith" and "Dark Lord" of them, is in all of the OT scripts - but it wasn't said in the movies - I wasn't sure if it showed up in one of the crawls, as it doesn't either ...

yeah, I always thought "The Sith" were a special order of "dark jedi", meaing like you proposed ben, that every Jedi can fall from grace and only the "darkest" try to find more power within old knowledge (aka Sith) ...

So if they as a group don't "walk around" in the PT, it's ok - but it would be interesting to show just how Vader became this "Dark Lord" ?