If diesel fuel is present in the engine oil, the nozzle of a leaky injector will look oily.
Disconnect the preheat relay connector.
Verify that this is not caused by an oil leak from the engine by checking the compression in the cylinders.
Check the condition of the glow plug: it must not be wet. Otherwise, replace the faulty injector.
If compression in all cylinders is normal, determine the faulty injector by inspecting the condition of the cylinders and pistons through the sockets of the glow plugs (Is the cylinder mirror greasy, are there any signs of overheating, has the piston been destroyed?).
If inspection of the cylinder and piston does not lead to conclusions about the cause of the malfunction, remove the nozzles and replace the one whose nozzle is oiled.
Note. Before replacing an injector, check that the O-ring is present and correct.