Alan Ratcliffe wrote
I always recommend Fret Doctor from www.beafifer.com for rosewood, ebony or pao ferro boards. Magic stuff - conditions and darkens the wood and lasts well. Once a year with that and just use conventional "Lemon" oil (Dunlop or D'Andrea) for fretboard cleaning every second string change.
[EDIT - Reinhard posted before me, so consider this post a big +1]
Looks like I've been using my lemon oil wrong ? I use that once a year for conditioning of the fretboard...