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The first thing I'd check is the crankcase pressure relief valve (PCV) these are sprung loaded and are only meant to 'burp' excess pressure every now and then. The exiting gas is piped back to the inlet , however should the valve stick open then the engine tends to suck the air out of the crankcase all the time and this carrys oil over with it.
If you remove the air intake pipe and look inside where the crankcase breather is plumbed in you may find evidence of oil.
If you do find oil replace the valve. If this is all OK then its possible that the valve stem seals are past their best and are drip-feeding oil into the intake. These can be replaced without removing the head by a competent garage in half a day or so
Also check inside the intake just after the turbo for any signs of oil as this would be an indication that the turbo oil seal is starting to fail and a replacement / rebuilt turbo may well be required. Have a feel of the turbine wheel - there should be no play radially at all - but a little bit of axial / end to end float is normal
an engine flush may also help if you've got a sticking oil scraper ring so its worth a go, as for the oil if you go up a grade / use slightly thicker oil then this is less readily drawn into gaps and less likely to be burnt off