I have used lemon oil for years on my bass fingerboards and applying lemon oil was one of the first jobs I did when I brought my brand new 6-string home last week. Here some tips for you though:
1. Only use it on bare wood (i.e. fingerboards) as it's no use using it on painted or lacquered areas (you need guitar polish for that).
2.
NEVER apply it directly to the fingerboard, place just a FEW drops onto a soft cloth and use that to apply the oil to the fingerboard. A little, often, is better than flooding the fingerboard in one go. A bottle of oil will last for many years if used correctly.
3. Let it sit on the fingerboard for about 30 seconds or so and then use another CLEAN cloth to wipe off any excess. The first time you do it the fingerboard may suck in all the oil like a sponge, if it does, apply a second LIGHT coat but no more than two applications at any one time.
4. I would never buy lemon oil from a guitar shop as you will end up paying through the nose for it. I get mine from the cleaning supplies aisle of our local grocery store. You get more oil for less money that way!
5. You only need to apply lemon oil 2-3 times per year depending on how dry your local conditions are. If the grain in the fingerboard is starting to look more 'open' or you are starting to feel the ends of the frets on the outside edges of the fingerboard it's usually a good sign that it's time to get out the oil bottle.
As I said I have been using lemon oil for years and my 18 year old bass fingerboard looks as good now as the day I bought the guitar. It is worth doing as it really does protect and 'feed' the exposed wood and prevents it from drying out and cracking.
Hope that helps.
but I paid my £3 (about $6 I think) so I am choosing to believe it was worth it 
