I presume you are talking about the D179 diesel engine ??
When started and running cold the carbon in the exhaust combines with water from condensation and produces an oily looking black discharge, known as "slobber" this clears when the engine reaches working temp.
If the discharge continues when hot it is more likely to be leaking valve stem seals, and typically there will be black oily smuts on the bonnet & cab if fitted ??
Valve stem seals can be replaced without a strip of the head !!