Oil in the injector harness is an ongoing peril with the Td5. The oil can be cleaned out of the plug with some petrol, brake cleaner or I would prefer iso-propyl alcohol. When you tackle the job, it's often a good idea to remove the white insert in the plug to clean inside.
Whether or not you open the ECU is up to you, because of the age of the vehicle you're unlikely to void any warranties and in the case of very bad comtamination the oil can find it's way in there too. If there is contamination in there, bear in mind that there's some sensitive electronics in there too, the best cleaning method is to just wash it out with an aerosol spray which readily evaporates rather that trying to remove it with paper towels.
Remember also that even though you might have cleaned all the oil out of the plug, there's still quite a quantity left in the wiring harness between the cylinder head and the red plug, so if you can spare the time it might be worth allowing some time for that oil to drain out. It might take a number of goes to clean the oil out completely.
Don't forget that the root cause is the oil in the cylinder head finding its way past the seal on the injector harness. That will also need replacing.
Sometimes the answer is easy, sometimes it's a FAFF
