What is the mileage on your Engine.
If you are burning Diesel Fuel and the Plugs are shiny and oily the Oil is coming from the Engine. Worn Piston Rings, Valve Stems/Seals/Guides.
Another possible would be the Turbo Seal on the Compressor side
With your description of the #5 Glow Plug not being Oily this does make it seem like it might be something coming into the intake manifold. Turbo leak or Blow-by from the Intake? Because where the Turbo outlet hooks up is closer to the Front end of the Engine than to the Rear.
If you are burning an alternate Fuel that is not Biodiesel than that could be the cause.
Since Winter is all most hear you might think of adjusting the Valves, checking the Camshaft Timing and Fuel Injection Pump Timing.
If the Plugs are not actually Oily (burned Oil has an oder to it) it could be your Injectors need servicing/rebuilding or you are having some issues that are related to the Fuel burning well in the Engine.