There is a lubricant called "Guitar Grease". From appearances, it is graphite suspended in hard wax.
I tried it on two nut slots, and I think it works signficantly better than soft (6B) pencil lead. Does anyone have anything negative to say about this stuff? To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
