Quoted:
I have put a bunch of barrels on uppers.  I usually use regular grease if that is what I have.  But I have put a few barrels together at a friend's house, and he uses brake grease because that is what he has.  I thinks it is "moly" but it might be graphite.  Not sure what the nature of the graphite-aluminum issue was.  This is what I found: http://www.finishing.com/82/53.shtml

For non-moving parts, a little graphite in the grease will probably not matter.  For moving parts where the anodizing wears, not as good.  But it appears that the problem is graphite is galvanic reactions.  Graphite is not a good lubricant by itself.  

FWIW.