I'm not sure about lemon oil, I've heard some people say it's not good for fretboards. As a former chippie I know that some woods benefit from being oiled - linseed oil on beech worktops is a good example. From what I hear lemon oil is a solvent/ degreaser which is a different thing altogether. I know it smells nice but you'd get the same effect using lemon fragrance fairy liquid, although it might look interesting if your fingers are sweating at a gig and the neck starts producing lather.
Two methods I've been recommended for cleaning fretboards are:
1. 000 grade wire wool - you'd need to put some masking tape over the pickups as they'll attract the minute metal particles and you'e supposed to do it very gently.
2. Gently scr@pe of the dirt with the edge of a credit card - I'd recommend using an expired one otherwise I can't predict what would happen when you put it in a cash machine.