A clogged crankcase breather is a very real cause and relatively easy to check. Having oil leaking out the air filter is a sure sign you will need to do something.
One question, did you check the turbo for oil leaking. Given the quality of the base D3 block, I have a hard time to believe the piston rings will be the problem unless the engine has had a very bad first life.
Leaking oil seals in turbo compressors are not uncommon.