The wood used in the core of a metal covered wrongko is never expensive wood, moreover, it is never seen, it does not matter in even the smallest degree if the oil penetrates the wood. In fact, it is perhaps desirable that the oil does get into the wood, as it will form a barrier between the cellulose of the wood and the ferric material of the blade.
Where oil stains do matter is where an expensive, sometimes irreplaceable timber has been used in an expensive, irreplaceable wrongko. If you cannot see the wood, and that wood is low grade wood in any case, it doesn't matter.
What does matter always is preservation of the blade.
In traditional thought, the dress, no matter how luxurious & expensive has no real cultural value. Throughout the existence of a keris, dress is changed frequently and for many reasons, so it is really only people who value money and people who value the art of the dress who care about the oil stains.
In traditional Javanese thought the replacement of a scabbard is seen as something desirable, in a way, something like replacing an old wife with a younger woman, again, something that is not at all uncommon in Javanese society.
Collectors try to avoid staining a wrongko, it detracts from overall appearance, & from value. The actual users of keris are usually a bit less particular.
But apart from all that there are the perceived feelings of the keris itself to consider. It is widely believed that the material used for a wrongko should ideally be a material that the keris itself will feel comfortable being next to.
Again we can draw a comparison with women:- just because we see something as beautiful that does not necessarily mean that the keris will perceive it as beautiful. One of the most highly favoured woods for a keris is scented sandalwood, this is a very plain wood unless it has some sort special grain, like simba (feather crotch), so giving a keris a scented sandalwood wrongko is like paying respect to it.
On the other hand, we might find ivory to be a very attractive material for a wrongko, but ivory is hard, unyielding, cold. The ivory wrongko is a statement of our own prestige, but we need to ask if the feelings of the keris are in accord with our own feelings, or if in fact we might be acting in a slightly disrespectful way.
Writing the above my thoughts were circling around things I was taught by Javanese teachers, but it perhaps does not hold true that a similar system of values would apply in Bali, or anywhere else outside of certain areas of Jawa.
I guess, in the final analysis it all comes down to one's own personal system of values.
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