Oil in the intercooler comes from two locations the CCV (Crank Case Ventilation) and the turbos Compressor seal. The air after passing through the air filter then through the MAF (Mass Air Flow sensor) enters a long duct where the crankcase ventilation enters the incoming airstream. The air then passed down into the turbos compressor inlet where the air is compressed. Behind the compressor wheel is a seal. The seal is not a positive seal and under certain pressure conditions (low boost pressure, coasting, idling etc) the oil can pass through the seal into the compressor housing and mix with the air going out of the turbo. After the air exits the turbo the pressurized air then passes into the lower intercooler inlet where the air rises up and out of the intercooler. If there is any oil in the air it tends to settle to the lower portion of the intercooler. For the most part once the car is on boost the oil in the intercooler is drawn up and through thus purging the intercooler of excessive oil build up. A car that idles for long periods will often see a puff a smoke the first time you floor it...thats the oil getting pushed out and ingested into the engine. After the air exits the intercooler the MAP(Manifold Air temperature and Pressure) reading sensor checks the temp and pressure for corrections for turbo and fueling control. After the air passes the MAP sensor it then enters the EGR (Exhaust Gass Recirculation). The EGR doubles as an exhaust gas injector and Anti-shudder valve. The anti-shudder valve has two functions, one to prevent shudder during normal shutdowns and second to act as an emergency shut off should the compressor seal fail and allow an uncontrolled volume of oil to enter the cylinders (this results in the engine running away and simply shutting off the key will close the valve and shut down the motor regardless of fuel or oil supply since the air is removed. As an emergency backup the engine has a vacume accumulator that stores sufficent vacume to close the valve and keep it close until the engine off cycle completes). air after the EGR and anti-shudder valve then makes a 90 degree turn down into the manifold plenum where the air divides and goes to its respective cylinders. Due to the design of the plenum/manifold should an oil seal fail the oil tends to collect in the two center cylinders (#2 & #3) because of the main air entering directly at those two intake ports.
Anyway thats the basics of the TDI intake system.
DB