Thanks for all the advice. A complicated topic.
I went to a major national guitar chain, looking for lemon oil. As it turns out, all of the brands that have "lemon oil" in the title, do not contain any lemon oil. Just flavoring. Truth in advertising.
I checked out WD-40, and was told not to use it because it has petroleum distilates. But a major brand of fretboard oil has a warning label that the product contains these distilates. Go figure.
I was told not to use olive oil, because is could go rancid. Someone used it, and left it in a case in the garage for a few months, and had a lot of fungus growing on his fretboard next time he checked. Keep it in the open.
Then the topic of using nothing. If you live in an area where the average relative humidity is less than 30%, you will need to oil periodically. On the warm Texas coast, we certainly do not have that problem. I have a piano built in the 1800's in Austria that is doing fine with nothing on the ebony keys that may have been lacquered a long time ago, but nothing since.
I found the Boyajion Lemon Oil mentioned earlier. I tested it on different types of plastic, and left alone, it will make some, but not all, plastic look like someone took a match to it. It is in a glass jar, with a glass eyedropper for a reason. It can also dissolve some glues used to attach fret-boards. It is not acidic, like lemon juice, but actually has a near neutral PH. So the issue is probably the chemical reactions Mica mentioned before. So, use as per Mica's instructions on Ebony.
Long story short, I blended 25% lemon oil with 75% F-one, and have great results. The mixture does not have a negative effect on the plastic I tested. It also cleans better than straight F-one and smells a lot better.
