My $.02 on my Mercedes Diesel I had that problem because the HOLE in the dipstick was NOT supposed to be there. A little piece of filter material was supposed to stop the for happening.
With diesel at 13:1 the compression is know to have blow bye on worn rings.
When you changed the oil was it BLACK AS HECK, should have been from blow bye and Diesel Fuel.
I assumed you changed the oil filter that is why you added the extra quart in the refill of the engine.
99% of PCV valves are in the Cam Cover at one end of the hose that goes up to the air intake. I would check the HOSE and the INTAKE connection for a blockage.
Good luck

thanks George,have cleaned filter at top of cam shaft cover and all hoses etc.
oil was black as heck and i did change oil filter.have just located a hose going from above sump to turbo and this is next to be cleaned.might also drop sump and see if any bad gunge in there causing a blockage.dont like to think rings and pistons but options are running out .fitz