Is it okay to apply Lem-Oil to the back of a guitar neck?

You'll want to use a polish for the body and back of the neck. For the fretboard, it is safe to apply a small amount of lemon oil and quickly rub up to condition the fretboard, as well as remove any dirt buildup. It should be applied, and wiped off quickly, because the oil should not be allowed to set into the wood for too long, as it will act like water and warp the wood.