Bflat5
Simply mix 50/50 WD40 and rubbing alcohol together and get a rag. Apply a little to the rag, just enough to wipe the strings down. Give it a few good swipes, then get a dry cloth and wipe it down again just to dry it off.
Hm. WD40 and fretboards tend not to have a long and happy relationship. Rosewood boards generally need a light dose of lemon oil once in a while to stop them drying out too much (say spring and autumn), but WD40....no.
Isopropanol alone works fine for cleaning strings (just keep it off the board). The lubricant in WD40 gets everywhere and sinks into the wood. Over time, over-oiled wood goes mushy, loses it's grip on frets and markers and either goes soft or breaks apart.
Over the years I've replaced one or two fingerboards for people who, having diligently applied an industrial penetrating oil, designed to help free corroded fastenings and dispel surface water to bare wood, ended up giving me a substantial amount of money to sort the mess out. :-/