This is a normal .5mm mechanical pencil, but it is loaded with 5b lead specially for the purposes of lubricating string slots, particularly nut slots. Graphite sold as pencil lead comes in a lot of varieties and is composed of graphite, which is a natural lubricant and clay, which is not a lubricant and is also mildly abrasive. 5b lead has the highest graphite to clay ratio that I have found to be commercially available. Not only does it work better as a lubricant, but it also makes for easier application. 
 
Make no mistake, the standard #2 lead used by most people for lubricating nut slots works just fine, but this 5b stuff does work better.