I don’t know the answer either, but here are some guesses. Let’s pretend the choices are no oiling, occasional oiling, frequent oiling, and then make some assumptions about the dangers associated with each choice based on the little I know.
Let’s further assume that big changes in moisture content lead to changes in size (too wet swells, too dry shrinks, and that these changes can lead to bad things.
Let’s assume that oil slows the rate at which water enters and leaves. Too much oil could thicken/gum, make small changes in surfaces that affect sound, tuning, “playability”. Certainly some of my better used oiling rags are pretty icky (I use raw flax oil) after a few days exposure to air, and I assume something similar happens in the flute…
So what does that get us?
No oiling. Terry’s recent data suggests that at least some woods absorb at least some oil, and therefore are likely to absorb at least some water. In theory at least, if the flute is kept humidified, and played regularly, then the water going into the wood and coming out again doesn’t vary much. This seems OK to me, but I’d bet that oil provides a modest hedge for those times when one forgets to moisten the sponge, seal the bag, play regularly, and since most of us are human, this is an argument for some oil.
Occasional oiling. Unlikely to do much good, or much harm if the flute is kept humidified. So why not?
Frequent oiling. Seems to me that this would be OK if the flute was stood up to drain for a long time post oiling to get rid of as much non-absorbed oil as possible. But I’d bet that for most people most times, there is surplus oil, tht isn’t swabbed or drained off, and that over time, there will be oil build up. This seems to me like a risk unless the oiling is very light indeed. (A few drops on a cloth max). Still seems simpler to humidify the flute, and to oil once in a while.
The first sentence may be the most important one in this post.
Hugh