Especially instruments made from fruit trees (e.g. Plum) should be oiled from time to time to close the pores in the bore-holes and to make the walls more resistant against humidity.
For this it is recommendable to submerge or — at least — wipe out the wooden parts with linseed oil. After the treatment is important to wipe the parts clean to avoid clogging by “resin-bleeding” linseed oil. In the worst case parts can not be separated, once they are connected.
Threading and key paddings should not be oiled to avoid sticking. If you just wipe out your instrument, you can protect the key padding with tinfoil or cling film. If you plan to submerge the instrument, you should remove all keys before the procedure.
Instruments made from tropical woods (such as Ebony, Cocobolo, etc.) are not treated with linseed oil. Due to their density these woods do not absorb the oil. This may cause clogging of the bore-holes. In this case, an acid-free oil with a low viscosity (e.g. almon oil) is a better choice.