Is this a diesel? The crankcase breather system feeds into the air intake and produces a oily deposit like you are experiencing. Mix this with hot sooty gases from the egr valve and it is no surprise that clogged inlet manifolds feature regularly on here (and indeed on forums for other makes too). Usually nothing to worry about and considered normal (within reason of course:smileUpon inspection I've noticed that the throttle body is a bit oily. Any parts that are know to sweat? I'd like to keep the engine clean.