Oil in the plenum is usually down to turbo.I,ve had this on mine found the turbo was leaking from the compressor into cooler and getting into plenum

Only way was to clean everything big job but worth it

You say its collecting around no 4 at the top? looking in around the spark plug hole or inside the bore? if the first around the hole its the cover gasket failed if the bore seems like the guides as Mark has suggested

Remember any oil in the combustion cycle will cause det so be carefull

Let us all know how u get on

Paul