Sludge/Oil residue under Radiator cap

I've been trying to track down a coolant leak from the front and back of the engine. I finally made it out to get a pressure tester for the cooling system, but when I pulled the radiator cap off it was covered in a layer of black sludge, kinda felt like chapstick. Coolank had a light layer of it floating on top as well. I gotta assume it's oil contaminating the system, but does this give any clues as to where my leak may be? The coolant leak is pooling on top of the timing cover and dripping down the backside of fengine. Maybe a lower intake manifold rather than a head gasket/bolt?

Thanks!
Jay