How long was that? if thats the case I'm not out of the woods yet if yours just started to show now...
It was out of the package about 3 days before appearing. And even so, it's just barely noticeable on the bottoms of the lower plates.
Mine was also about 2-3 days before really showing up, and it started out minor, wipe it away and i could go like 4-6 hours without any sign again, the more I kept cleaning it though, the quicker it'd come back.
Now it's been like this for days since I stopped wiping it away, with no change, other than oil starting to come out of the bicep plates as well.
I just received my 2nd Batman DX figure and it has the oil as well... I guess it's inevitable for now.
At this point, I'd only be surprised if a figure didn't show it. Something about the material used for the ab piece of the shirt and the bicep plates on the arms is prone to this, either the nature of the material or something that was done in the production process.