Thanks.
A few years ago I knew a professional bookbinder/restorer.
He used olive oil on the leather after the book was bound.
He said it was the best treatment he had found for the leather.
IIRC -- Yvsa reported here that using olive oil
as a blade treatment was a big no-no.
It got gummy.
It may be that when it is incorporated into
leather, horn, wood, or other material
it behaves differently.
Or is just protected from oxygen.