I know this topic has been done to death a bit, but I wanted to share my latest experiences.
I have suffered from this on and off for years and it has been either broken turbos or the faulty oil filter issue.
Since having my new K24/26s fitted, the car has been perfect. I took in in for an oil/filter change and....... yet again it starts smoking on start up if left for any length of time. The shop tried a different oil fiter, still smoking
I have just had a kit fitted (made by/for RS Tuning) which is basically 2 valves, one coming from the oil to turbo inlet which will only allow the oil through over a certain pressure, the second is from the turbo sump to the scavenge which stops oil running back from the scavenge to the sump. Apparently this kit mimics what Porsche have done with the 996tt to good effect so Porsche obviously know about this problem and my source says the latest word from Porsche is not to run with the oil level over 1/2 full on the guage which apparently makes it less likely for the oil to seep through the system into the turbos when swutched off.
The kit I had costs about $1500 but I have since seen that Cargraphic do a similar kit which looks a little cheaper:
KKKP93ÖLKIT
Kit with non return valve
Price: 585,00 EUR /5

 

andial has one which is even cheaper although not a valve but high arch oil line to restrict flowback.

 

I put the Andial ones on my car and I thought that is what kevin insisted on using when
you install his turbos???
MK