When was the last time you cleaned the air intake pipe and the throttle body?
Has the oil accumulated as much as it has in such a short space of time? if so then there's problems.
If its built up gradually then this is normal.
As for the power i would say this could be the build up of oil causing blockages in the intake manifold.
Does the car sound rough at idle and when revving?
The EGR valve aswell may be knackered so it could be a good idea to get this replaced.