Having just been in the presence of greatness to rebuild my box, i can confidently say that the only thing that needs doing is the needle roller bearing needs to be changed (5th). Low oil is fine briefly, but if it whines, it means that the plastic needle roller bearing cage has melted. You can get to it via the metal side cover without stripping the entire box. About £4
oh, and while in there, swap the circlip shim for a new thicker one to take up any slack from wear.