Believe it or not, the most important thing about greasing the joints is not the composition of the grease, but that you successfully keep air, and thus oxidation out of the joint. With the small distances between the rail and the joiner, having actual conductive grease (electrically conductive) is nice but not necessary.
All the graphite-based "conductive" greases are not conductive, which you can prove to yourself with an ohmmeter... (there are some real conductive greases, usually with nickel, copper or aluminum in them)
Greg