just to add my 2cents, diesel engine oil ALWAYS gets black very quickly, trust me i have over 1millions miles in diesels,also coolant could leak into intake manifold if manifold has water jacket,ask bmw as i had 318ise losing coolant and had to find specialist engineer to locate problem. he did "sniff" test to locate hydrocarbons in coolant although he knew it was not cylinder head gasket,it was rubber seal on intake manifold.not a major problem if using small amount of coolant but check level frequently.