Pop the turbo off, inspect the bearings and see if there is any evidence of low oil flow to the turbo. It could be something as simple as a piece of debris or dirt in the line.

 

Sounds like a real waste of time, the oil is filtered.

 

I'm just speaking from years of Turbo car experience. It may or may not be the issue.

 

If the oil in the turbo overheats it can coke up, and if any of it breaks loose and gets into the bearings, bye bye turbo. Oil filter doesn't do anything for you here.

 

Turbo is water cooled to reduce/eliminate coking if shutting off too soon the water continues to syphon thru cooling it. It is still a good idea to let it wind down and cool for 30 seconds before shutting off after a hard pull.