A penetrating oil should be applied to the wood parts every couple of months.
These oils soak deeper into the wood than do the lemon or orange oils
Tung oil, gunstock oil , are usually applied twice with a 24 hour wait between applications. These penetrating oils get inbetween the and into the wood grain itself. where as the non penetrating oils only fill in gaps around the wood grain.
Before applying these oils , pre clean the wood with lemon or orange oil
Apopular oil for the dark woods of a fret board is virgin olive oil.
The best advice is to oil the woods often , after each use is not excessive at all.
If the wood dries out, all sorts of problems develop, ,
for one thing the frets will popup or become loose,
Without oil the woods deteriorate ,