Actually, the book of Geert regarding the musical side of things is really, really excellent!
That being said, all his comments on care and daily use are absolutely off the wall and nonsensical, according to professional makers, with Geert claiming to know more about recorder making than the makers do…..!
Fact is: a thin coating of oil on the outside of the recorder cannot possibly reach the interior. Whereas, when you coat the interior, a micro layer builds up to assist in increasing the instrument’s resistance to moisture.
It is not even a question of getting oil into the wood, it is a question of getting a layer on the wood. This was also Bob Marvin’s and Adrian Brown’s standpoint. (read Adrian’s book on care, top notch) !
With a good maker, the wood has sat submerged in oil for months before it even gets put on the lathe, and will normally always retain an amount of oil trapped inside it. Extra oil on the outside and inside form protective barriers to hold this in.