You could put a cloth on the card to see if someting is dripping from someplace or place a piece of cardboard there and then run the Pc for a day to see if something is in fact dripping or leaking. Without actually seeing it there's no wat to tell if it came from inside the card or not. If the oil spot is on the back of the card then it most likely didn't come from inside the card and it may have just been there from the factory and you didn't notice it when you first got it. There are several possabilities and your going to have to leave the card in the slot for at least another day to see if any more oil is there or if it's just an odd occurance from the factory.
The card is running ok?