I rarely encounter the need for fretboard oil, and lemon oil has done the trick when needed, if I haven't been lazy and simply reached for the thinners always sitting on the workbench. I'd like to try mooman's almond/vitamin E oil though, sounds worthwhile.

That fretboard grunge can be lightly(!) scraped off with a soft(!) piece of plastic so as to not scrape the wood before finishing up with a solvent/oil. Lots of fretboards get too much crap to simply be dissolved quickly.

Peace, Mooh.