Caring for your shakuhachi
Caring for a shakuhachi is rather simple when compared with many other instruments. They are incredibly durable and have no moving parts to worry about. Proper storage and cleaning are the most important measures for maintaining a shakuhachi. It is advisable to store your shakuhachi in an air-tight bag or case because bamboo can crack when there is a large enough shift in the humidity.
Generally speaking, around 50% humidity is desirable for preventing cracks. If a shakuhachi should crack it can almost always be repaired without any effect to the sound. To help prevent cracks, avoid leaving your shakuhachi out of its protective bag or case especially when you find yourself in harsh dry environments such as; AC, heaters, camp fires, closed automobiles, deserts, beaches, snow, and so on. Too much humidity can be as detrimental as too little so it is best to regulate humidity. One elegant solution to this are silicone humidity regulating beads.
As for cleaning, the inner bore of the instrument requires periodic attention because this is where moisture from the breath can condensate. To clean the inside of your shakuhachi, pull a swab-cloth through the bore after playing it. If you do not have a swab-cloth you can make one by tying a length of string that is longer than your shakuhachi to the corner of a bit of cloth which is of a size that will not get stuck in the bore. To use the swab-cloth drop the string down the bore, protect the edge with your thumb, and pull the cloth through. To help the string drop through the bore more easily you can tie a small bead to the end.
It may also be necessary to periodically clean the inside with a one-to-one solution of water and distilled white vinegar in order to kill mold, germs, and remove odors. To do this, moisten the swab-cloth with the vinegar solution and ring it out before pulling it through the bore then wipe off any solution that gets on the outside of the shakuhachi. The vinegar smell will completely dissipate in a few hours or less.
You can clean the finger-holes with a baby size toothbrush, cotton swabs, or nylon bristle pipe cleaners. For a natural unlacquered bore you may need to use a dryer vent brush to periodically scrub the bore should it become moldy. To do this, dip the end of the dryer vent brush in the vinegar solution, protect the edge with your thumb, and carefully scrub the bore. When you are done you can run the swab-cloth through the bore to soak up any excess solution.
Lastly, I do not recommend oiling shakuhachi. It can possibly help prevent cracks in shakuhachi but very few oils will actually work and they have to be applied expertly. Furthermore, these oils have strong odors which I personally do not find pleasant. If you oil a shakuhachi it will most likely prevent it from being able to be lacquered. I also may turn down returns or trades if a shakuhachi of mine has been oiled.