Lemon oil, in the context of guitars, doesn’t have much to do with lemons. There are lemon oils which do, used in aromatherapy and similar applications, and others produced for furniture. The guitar stuff, though, is a mineral oil, perhaps with a tiny dash of lemon oil and possibly some colouring too.
Why does it matter to me? Only a little over three decades into my playing guitars and basses, I’ve finally ordered a bottle, which I’ll use when I change the guitar string that snapped on my yesterday. I’ll be interesting to see how it does.
A note of caution. Apparently it shouldn’t be used on maple fretboards but, now I think about it, I don’t have any of those! I did, once upon a time, but that bass got given away a long time ago now, so nothing for me to worry about.