I asked Hans about this...Hans is a factory trained Mercedes meister engineer. He said you should never have to check this if the car is a later model than the 220D. All the later pumps are fed oil from the engine and back out again.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM