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.
Seals are fitted at both ends of the impeller shaft. The primary purpose of these seals is to seal the high pressure gasses (exhaust) or air (intake) from entering the center housing and into the crankcase. The secondary purpose is to prevent the oil from entering the housing.
Traditional turbocharger design employs a conventional plain bearing that runs on a film of oil. This is known as a floating metal bush. Oil is fed through the bushes and forms a cushioning layer between the turbocharger shaft and the supporting bush. The shaft relies on a constant supply of fresh, clean oil over a very wide contact area in order to maintain sufficient clearance from the bush itself.
A more modern and efficient design utilizes ball bearings at either side of the turbocharger main shaft instead of metal bushes. These again are fed with engine oil, but no longer rely on a thin film of oil over a wide area to support the turbocharger shaft.
The result is an outstanding reduction of frictional torque on the rotating turbocharger assembly in contrast to the old fashioned floating metal bushes. The improvement in turbocharger response, particularly in the lower to mid turbocharger speed range is phenomenal.
If the bearings of the impeller shaft are worn the shaft does not rotate in a complete circular pattern and the oil seal fails to isolate the gasses/air and oil. Worn bearings can be due to high engine mileage, contaminated engine oil, incorrect engine oil, or inadequate lubrication or cooling.
Worn bearings can be checked by checking the play on the rotor on the X and Y axis. There should be no play. A small amount of play on the Z axis is acceptable.