How to fix oily modeling chocolate
Having made the same mistake I will try and answer best I can. 

Use to melt chocolate in a steam jacked copper pot. With a mechanical stirrer, witch stirred a 1 rev a second. It happened once that the chocolate seamed to created a layer of oil on top. I then saw a fault on the steam pot valve that to much steam came trough so making the chocolate too hot. 

Would not say its the way you stirred but just too much heat. Chocolate should only be heated up until it melts, and all your ingredients are properly mixed. So to fix it? Add one  tea spoon of lecithin. and boil again. That's what I tried and it worked.