WD40=very very bad
Vegetable oil=very very smelly and molding later on=bad but not too harmful. Easy to clean off
baby powder=bad, hard to clean off, and can eat into the plastic
Vaseline =unknown as of yet, hard to clean off, no reports of damage to plastic, but damage to metal parts of cube-springs and screws
graphite=ok, seems to turn better, but no report on damage. hard enough to clean
cube made lubrication=bad! makes your hands smell!-just my idea. no comment!