IMO, half or one teaspoon of oil would not cause any harm. For drier climates, a teaspoon a few months apart helps. When playing in India, i did notice that the bat does loose moisture and a teaspoon of oil always helped to get the response back. I usually oiled only the face and not the back or the toe. The wickets are very dry in India and on the bats that I oiled the toe got damaged more from tapping than those which were left un-olied. Just my experience.
Here in the UK, I use half a teaspoon before knocking it in and possibly half a teaspoon at the end of the season to protect it during the winder from drying due to the central heating.
Oiling the toe in the UK makes a lot of sense as wickets are moist and invariably when tapping on the wicket some moist soil sticks to the toe. Toe guards help but they also hide any toe damage which only becomes visible when it has gone really worse.