Do NOT use "lemon oil" on your fretboard. It is, as mentioned above, a cleaner. This means that it will actually strip your wood, requiring more frequent treatment. I would also stay away from mineral oil. If you feel the need to treat your fretboard, then only use 100% boiled linseed oil. This is far and away the best treatment for wood available. This is proven by the fact that we have flintlock firearms that are hundreds of years old, and the wood is still in perfect condition, even though the gun was neglected for a hundred years. When the gun was built, it was heavily treated with boiled linseed oil, and the wood has/will stand the test of time. I have seen maple, rosewood, walnut, ebony, purple heart, etc all last with treatment with boiled linseed oil.
Personally, I will treat my fretboard every time I change my strings. Of course, I'm of the Jamerson school of thought, and will never change my strings (unless one breaks, but I still won't change the whole set).