Dear paulgill:

Do not use the regular (BLUE cap) Mobil 1 synthetic oil (any grade)
in your 300SD diesel car. If you really want Mobil 1, you need to get the RED cap Mobil 1 oil bottles, because it contains the necessary soot control and detergent additives for diesel engines.

Castrol Syntec (any grade) is not perfect for your car either, even though it may have CF spec. The newest diesel oil spec is CI4.

I think the best synthetic oil for diesel cars is Delvac 1 and then Shell Rotella synthetic oil. They both have CI4 spec and are designed specifically for diesel engines.

Best regards,

Eric