External lube is still required on a sealed chain to keep the rubber X/O rings in condition, to replace internal lube that's lost through normal use, and to lubricate the rubbing surfaces of the chain.
WD40 is not recommended by either chain manufacturers or motorcycle manufacturers, as it disolves the internal lubricant and encourages it's escape from the chain, and the WD40 is so thin that it flings off a moving chain far too easilly making it ineffective as a lubricant.
Check the chain & bike manufacturers websites/owners manuals for more information.
Matt
