Oiling guitar fretboards made of rosewood or ebony will help keep the wood from cracking and prolong the life of the fretboard --- as well maintain an aesthetically pleasing appearance. This is because rosewood and ebony fretboards are unfinished and susceptible to drying out. This is not the case with fretboards made of maple, which are usually coated with a varnish finish and should not be oiled.
Determine whether your fretboard is made of maple, rosewood or ebony. A maple fretboard will be light in colour and shiny because of the varnish. A rosewood or ebony fretboard will be dark brown or black, and because there is no varnish, the wood will have a dull finish and the grain of the wood will stand out more than with a maple fretboard.
Clean the rosewood or ebony fretboard with a slightly damp rag after first removing the strings. There will probably be a build-up of finger oils around the frets. You can sometimes remove this with a fingernail. If the fretboard is still dirty after using a damp rag, gently rub the fretboard with #0000 steel wool, focusing especially on the areas around the frets.
Wipe the fretboard clean with a dry rag, making sure to remove all loose dust and debris that may have accumulated from the steel wool.
Apply a small amount of 100 per cent lemon oil to a clean rag. Wipe the fretboard with the oiled rag making sure to cover all the exposed wood. Do not over-oil the fretboard as too much oil will make it difficult to play your guitar.
Wipe down the fretboard once more with a clean, dry rag to pick up any excess oil that may not have penetrated the wood, and then replace the strings.
Always try to use the least abrasive method of cleaning first and only move on to the stronger method if absolutely necessary. Only use steel wool on a rosewood or ebony fretboard; steel wool will damage any finished wood. Only use 100 per cent lemon oil. Do not use furniture polish, such as Lemon Pledge, as it contains waxes that can damage the wood on your fretboard.