Thought I'd try it out. This coin was extremely dark brown when I dipped it in the oil. It actually looks pretty good in the right light, maybe another year?!
I applaud your patience. Ever seen this site? http://www.metaldetectingworld.com/cleaning_preservation_coin.shtml
Honestly, I think it would look better without the olive oil treatment. It just looks cleaned, ugly IMO. Some coins can't really be helped without making them worse.