Just use an instrument polish cloth, it picks up most oil and grime. Use your thumb nail with it along the edges of the frets - it's easiest to do this when you have the strings off. I would avoid using any sort of cleaner, I find that anything besides my own sweat on the fingerboard just slows my playing down.
i think what he is referring to is fretboard cleaners/conditioners. there are several products out there that are made for guitar to clean and condition your strings and the fretboard. the way i see it, guitar fretboards are made with the same materials as ukulele (most of the time... rosewood). so if it is safe for a guitar, i dont see why it wouldnt be safe for an ukulele. my brother and i use one product and it has given us impressive results on all of our stringed instruments. from steel elec guitar, to steel acoustic guitar, to nylon acoustic guitar, to ukulele.