My Ibanez SAS32EX has been in regular use by me for around a couple of months or so now, and its fretboard is FILTHY. I've been giving it small cleans every now and then, but I'm not sure why it's so disgusting. Almost every fret is completely covered in what I can only assume is skin. Is there any good way to clean a guitar's fretboard without hurting the natural oiliness of the Rosewood?