That is some nasty crap to be moving around at umpteen squillion revs and then putting into any engine, just guessing, guessing engine rebuild is part of the full rebuild too for whatever reason.
That's not surprising, judging by all that wholesome meaty goodness that it's been ingesting.
As said above, main suspects are the breather and the turbo bearing's seals. It's worth giving your intercooler a good clean out too, with petrol or paraffin to dissolve some of the muck.
check for blocked cyclone as well - where the breather exits the rocker
with engine started take off the oil filler cap and see if its chuffing out of there also.
My old 19j looked like it had a second exhaust coming out of the filler cap by the time i was finished with it and the chassis leg was barket with oil that had come through the breather and out the filter valve
Disconnect the breather and put your hand over the hole. A little air movement is not unusual but if it seems to be blowing, maybe there's some loss of compression into the crankcase.