That is the oil vapors from engine blowby being sucked into the intake manifold. A nasty mess always. Another reason I like to vent the blowby to the atmosphere rather than the engine. I prefer doing that even though sucking a vacuum on the engine block helps to seat the rings better. Drag cars will often put a large fitting on the valve covers and run a hose from there to the exhaust collector to scavenge more vacuum inside the block for more power. Too bad catalytic converters don't like oil.