If you are sure that the engine oil is coming into the combustion chamber, then you have to overhaul the engine. After the overhaul is done, have it calibrated and and clean the exhaust pipe to get rid of the black smoke. And! remember, for diesel engines a big black smoke will always come out the exhaust if you suddenly step on the gas too deep and then over-revving it. (I mean always, that's why roadside emissions people are so stupid when stepping on the gas, they always step on it suddenly and over-rev it, it's a sure fail)