I know that one person’s experience does not equate to hard facts so feel free to take this at face value but I’ve used pure lemon oil on my fingerboards for over 30 years with nary a problem. I don’t use much - literally 2-3 drops spread evenly and then the excess wiped off after a few minutes every couple of years - but even if I did, I just don’t see how the acidity of lemon oil could harm a tropical hardwood fingerboard.don't use real lemon oil! it's very acidic.
For whatever it’s worth, pure lemon oil was recommended to me by Carl Thompson, who’s used it on his fingerboards since the ’70s.
I’m not saying I’m right or that there aren’t any better choices but it’s worked fine for me since about 1989 (and I’m still working on my first bottle!). No damage to board or finish, no loose frets, nada.