I know this is a older thread but I was thinking, maybe in this instance olive oil would not be wise to use, sure it wouls darken the patina but as already been mentioned it is slightly acidic which can only lead to problems with a already worn coin. If you did want it a darker tone, without using repatination products like Dellers or Jax then how about using a product like Renwax. Not only will it protect your coin but it will darken it slightly and once its dried and maybe after a couple of light coats you could go over the high lights of the coin with a cotton bud, buffing gently which will bring out detail too.
Just a thought, best regards romeo
