I thought you meant molded chocolate candies. You don't need corn syrup for that. If you mean chocolate clay, did you let it rest overnight to gain the right texture and consistency? Your recipe is what I use and it is greasy when it is warm, but once it sits it is not.
Here's a recipe on CC that uses 10 oz. of chocolate:
http://cakecentral.com/recipes/1608/chocolate-clay-modeling-chocolate
One little trick I learned in a Mike McCarey class - when combining the corn syrup (warmed gently to the same temp as the melted chocolate), stir only until it comes together and looses its shine. Be sure to scrape down and incorporate the contents of sides and bottom of the bowl and use a folding motion. Once it has a matt appearance, quit stirring!!!! Scrape it onto a piece of plastic wrap, wrap it well and let it cure at least 6 hours. Then knead the hard slab into a workable clay. The oily separation is from either overheating or over-stirring.
.Put it in the fridge for tonight only. You store at room tempature but I believe you're suppose to put it in the fridge for the 24 hour period. Your receipe should tell you whether you shoud or not 
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