Check the fuel filter element.

If it looks black then wipe your finger on it & black residue appears on your finger then it is engine oil.

Where might you ask does that come from!!??!!

One of 2 areas.

1) The Bi vacuum / diesel pressure vane pump gasket or pump internals. (Always check this first)

or

2) Internal cylinder head failure. This engine uses internal fuel delivery galleries within the cylinder head to supply the injectors. The injectors are a Pumpe Duze type that uses a cam lobe to create high injection pressures. This design suffers from injector rock from the cam causing the cylinder head fuel galleries to fracture & leak engine oil into the fuel & vice versa within the cylinder head. In this case a new cylinder head will be required