Do I really need a new turbo?

I have a 2013 3xi28, on occasions the Drive Train Malfunction light appears,
usually on exceleration or incline and at times may loose some power, but
not always. I have taken it to the BMW service department, it was
reprogramed , they drove it and it drove fine. Five miles down the road and
$466 for oil change and reprogram the drive train malfunction light appeard
again,( no loss of power) I called and was told usually when this happens it
needs a new turbo. Est. cost $4000 & change. Then someone told me there
is something that mimics this and when it does occasionally a black or blue
smoke appears on start up and that has happened a few times. They say
that oil backs up in the line. I do not want to spend $4000 if not needed. So
why isn't it showing up in diagnostics that it needs a turbo? they told me all
codes are stored in the car but it did not show any problem with anything
involving a turbo issue or any problem at all with the car. Please help .