Re: For guitarists of all levels...
No, no, not cooking oil - it'll get rancid and sticky in time.
Actually oiling the fretboard is purely a cosmetic tweak. It does not improve the sound, but depending on the condition of your fretboard, may improve playability slightly. A well-made fretboard only needs to be kept clean.
I have had good results with lemon oil. I need one drop for a full fretboard, so a little goes a long way. Before I got that, I was using Vatika. Again one drop for the whole fretboard.
Only use an oil that you would apply to your skin or hair (not cooking oil like mustard), or a synthetic oil like Vatika or Jasmine.
What works best is the regular dishwashing scotch brite, used dry, in the direction of the grain. It also cleans the frets. Oil will restore the original condition and finish, but is not strictly needed. The oils from your skin will transfer to the fretboard in time.