I don't use lemon oil on the fingerboard. Only the top, back and sides.

As for tung oil (here is some more info) you have to use it very sparingly, and once a year is enough. You apply it lightly and wipe it all off starting about 30 seconds after you apply it. If you leave it on longer than that, it will make the fingerboard very sticky and it's a bitch to get off once its gets to the "sticky" point. I wouldn't have known about it until the luthiers at the vintage shop I deal with swore by it and instructed me on how to apply it, and they reiterated what BOB said about NOT using lemon oil on the fingerboard. These guys are extremely knowledgable about luthiere and guitar maintenance matters, so I felt I could trust their expertise. They've never steered me wrong about anything.