The biggest cause of these turbo,s to fail is lack of oil changes and heat soak. The 1st is self explanatory the second is caused by the design of the turbo and a simple mistake that most people make. There isn't a bearing that fits tightly between the cartridge and the casing . there is a gap in the thousands of a millimetre this gap is filled with engine oil and in theory the turbo can run forever as no wear will occur due to a supply of fresh oil. However these turbo,s have to work really hard and run very hot. if they are switched of when hot then the oil that sits between the bearing suffers from heat soak were the oil soaks up the heat and starts to burn. This creates carbon deposits which then contaminates the oil system. Do as paul suggests and remove the filter in the feed pipe then give the engine a oil change and a flush. when you've finished a long/hard journey let the engine idle for a min before switching it of as this allows the oil to cool and stops the burning. Hopefully you've caught it in time and a few flushes will clean it for you. Have you just bought the car or had it a while so that you know the history of the turbo/engine