Sajin
RKarov
Here is one more. 
That is nothing to worry about. Oil residue from thermal pads isn't a problem.
You can see the pads here...
I was taking a picture for someone else and noticed the "slime" left on my stock TIM still.
Those pads are
Seriously, that bubbly looking stuff, stock TIM pad grease.
Oh and lol, my point, stains would probably be inevitable, so yea, no worry.