PCV and oil separators are in the valve cover. When the throttle shuts on decel, it can suck too much oil into the intake ports if the PCV vacuum limiting system inside the valve cover is bad. It's not cheap, and you will need new injector seals (and the proper tools to install them without damaging them). Much cheaper than new turbos, though!
Valve stem seals could be bad, but how many miles on your car?
What oil are you using? How long do you go between oil changes?
What color is the metal inside the engine (look inside the oil fill cap)? Brown, tan, gold, or shiny metal?
E92 335i/6MT Dinan Stage 3: intercooler, oil cooler, intake, flash, exhaust, LW dual-mass flywheel.
Vishnu PWM methanol custom trunk mount, Diffsonline 3-clutch Salisbury custom diff, Ohlins Road and Track coilovers, Vorshlag camber plates, much more...
E28 M5 w/ buncha stuff, R171 SLK350