There is no residue inside that is holding the box together, the residue is a build up of varnish usually from old oil, this can affect the general performance, particularly with shifts.
Detxron oil goes brown due to age and contamination, it will do no harm to change it, it also will not stop a fault the box may have.
The boxes do have a few probs as they get older, most notably diaphragms going hard and speed issues affecting one gear only.
tabetha