Do you guys have that on your thermostat? Every time I reached down there's always black sludge around that area. I wiped it clean and tried couple weeks later and there's new sludge again. It's not really that bad but just curious.

 

oil leak?

 

See the red circle area, that's where I found sludge.

If you guys don't mind, wait til your car cools down, and reach down there... It's kinda hard to see with the coolant reservoir blocking the view.