Engine oil under pressure enters the bearing housing of the impeller shaft from the oil inlet side. This oil passes bearing and the wheel journal surfaces lubricating and cooling the moving components. Here it depressurize and flows by gravity through the oil drain line and into the engine sump.
Faults in the crankcase ventilation system can result in positive pressure being built up in the crankcase and restrict the oil from flowing down into the oil sump. This restriction allows the oil pressure to build up in the turbocharger which then allows the oil to escape past the oil seals at either end. The oil then enters the intake tract where it is sucked into the combustion chamber or it is blown out the turbine side into the exhaust.
All engines produce crankcase blow-by gasses, gasses which are forced past the piston rings into the crankcase. If pistons/piston rings/valves are worn the pressure in the crankcase can increase enough to push the oil out through the seals/gaskets. Under high levels of excess pressure the valve stem seals are subjected to a much greater load. As a result, oil is forced along the valve guide into the exhaust or intake. Increased pressure in the crankcase can also be caused by a defective positive crankcase ventilation valve. Check crankcase vents and breather lines to ensure they are free from any restriction.