what u can do it to do a compression test for each cylinder. check the compression and from that you can know whether your piston ring in good condition or not. Could be your oil ring is kaput and leak thro the combustion chamber.
Check spark plug and ur piston... wet or dry... if dry and full of carbon then i ok. if wet ,the could be a worn piston ring or valve seal is leak.
if everthing is ok in the cylinder, then can narrow your investigation at turbine area.if the turbine problem , could be the oil plate stopper at turbine wheel is broken and oil pass thro with exhaust... could be...
That my opinion and pls do correct me if i'm wrong...
Rgds
