I use a lot of boiled linseed oil on wood that will be outdoors, shovel handles, exposed cedar etc. Had never thought about using it on a fretboard.See if you can find bore oil. It's a woodwind product that's made safe for unfinished wood. That or boiled linseed oil, also highly recommended.
I wouldn't use lemon oil unless it's pure.
I am searching the forum and keep finding (Sticky Nitro neck syndrome), so it looks like a fairly common thing. I guess there are a lot of things that can react with the finish.