Common issue on the S54 engine is the CPV O-ring leak.
If you noticed an oil leak under the car, where the engine and the transmission meet, this is most likely the culprit.
It could be a rear main seal or the valve itself, but more commonly it is the O-ring that needs replacing.
O-Ring is made out of Viton® premium rubber. Direct OEM Replacement
O-ring is sold individually.