There are quite a few products available. Most people on here recommend Lemon Oil, but you should make sure it's the type recommended for fretboards, like this:

https://www.stewmac.com/Materials_an...Lemon_Oil.html





As a side note, I've actually just been made aware of the 'Great Lemon Oil Debate' via a Hagstrom page I'm on and there are people who are adamant it's a bad thing. There are some different sorts available and allegedly it can cause problem if you use the wrong one.

I am yet to have anyone provide a good example of how using the wrong one causes any problems. I think its all a bit of a beat up.

Personally, I think any furniture finishing oil should be fine. They are all designed to do basically the same thing, protect the wood and enhance the grain. It's one of those things with guitars there seems to be a bit of voodoo around.