There is no 5W-30 Mobil 1 approved by Mercedes.

You should be using 0W-40.

The statement 10W-40 is "too thick for winter use" is baloney as regards Mobil 1, as when one checks Mobil data regarding cold poor points all versions of Mobil 1 save for 15W-50 show the same number. Nevertheless there is no Mobil 1 10W-40 approved by Mercedes either. As for Castrol it's not a synthetic despite its labeling and isn't approved by Mercedes either.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles