FYI, most people don’t realize that a leak in the turbo is usually an installation/drainage or engine problem (such as excessive crankcase pressure) and not a problem with the turbo.
Installation/Drainage: Oil passing through the turbo must flow to the engine by gravity. Thus, any restriction in the oil drain line will cause the oil level in the turbo bearing housing to rise above the oil seals and result in oil leakage into the housings. ***This is one of the main causes of a leaking turbo.
Engine Problem: Excessive crankcase pressure (or #blow-by) can force the #oil to go through the turbo seals. Excessive blow-by is generally a result of a problem in the cylinder itself or a plugged crankcase filter. In an engine with blow-by, as ignition occurs combustion pressure escapes past the #piston rings. This blow-by causes excessive #crankcase pressure this pressure escapes up the turbo oil drain line and forces the oil out of the turbo seals, which will cause a turbo to leak oil.
This video below was posted by the awesome guys at @patriotdiesel, it shows a simple way to test for excessive crankcase pressure. To test, all you need to do is unscrew your oil cap and lightly power brake the engine to bring up boost, if the oil cap dances you know there is excessive crankcase pressure.