I used Formby's lemon oil for decades. Still have half a bottle left in the back on a shelf somewhere. Yes, commercial "lemon oils" are really just mineral oil with lemon scent but they work well enough and they don't build up. When buying linseed oil for fretboards always be sure to get boiled linseed oil, not raw. Raw linseed oil dries very slowly, tends to feel sticky, and over time it can polymerize into a varnish. Boiled linseed oil contains penetrating & drying agents to prevent such problems. I only treat my rosewood fretboards at two year intervals unless one really looks dry - which is rare. Every three years for ebony (which has tight grain) and Brazilian rosewood (which is naturally waxy). More often than that is unnecessary. Oiling is really more a cosmetic thing than actual conditioning. More importantly, I don't let my guitars become dessicated. Actual dryness is a serious issue that's completely separate from lack of sheen.