I am considering a 400E having done 210,000 km. Checked spark plugs I found that the color of the electrode was good, i.e. b but the threaded part had oil on it. Had a test drive on the highway the acceleration was ok. I wonder if the oily thing is the indication of piston rings replacement or just the valve stem seals. Hopefully it is the latter.

Can anyone tell me at what mileage should we think of a piston rings replacement?