For this you can use the method that has been used for hundreds of years.
To do this you need a hammer, 3 or 4 walnuts , and a nice piece of cloth
Using the cloth, gently apply the oily cloth on the duduk.
You can use this cloth several times until the oil on the cloth starts to dry out.
Walnut contains natural minerals, vitamins, and oil , this will preserve your instrument for many years. you can also use almond oil, or any other neutral, non-vegetable based oil (vegetable oil will go rancid on you).
The oil keeps a small amount of water in the wood, and helps keep extra water out, but what do the vitamins and minerals do for a piece of wood that’s been dead for maybe a hundred years?
I’ve been using the crashed walnut’s oil technique for about three or four months and it works very well.
I’ve used it for the duduk’s body and also for the shakuhachi ( especially the mouth piece, utaguchi, and the root’s area).
So far I prefer this kind of natural oil over the almond oil that you buy in shops.