Olive oil actually does work. Avoid things like canola oil though as those can get sticky. Olive oil doesn't get sticky and doesn't soak into the wood to fast so it does work in a pinch.
Boiled linseed oil would be my oil of choice though. Its basically the same thing as lemon oil minus the scent and usually half the price. You can find it at most arts supply and hardware stores and a $5 bottle will literally last a lifetime or a million guitar necks.
Oil is pretty much oil at the end of the day. It can be no name linseed or some "uber" oil (probably linseed lol) with a famous brand name on it at 5x the price and 1/5th the size. No one is really reinventing the wheel here.
Whats more important is not applying to much, letting it sit just long enough to give the wood the revitalization it needs and then wiping it off and buffing it well with a clean soft cloth, and most importantly of all, not letting it get down into the frets.
Work slowly and carefully. Qtips are my tool of choice for application. Even with the the most expensive and highly regarded of oils, you can screw up your fretboard if you use to much