Date registered: Oct 2009
Vehicle: 2007 GL450; 2000 SL500 (AMG Sport); 1999 CL500;
Location: West Michigan, USA
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I strongly believe that synthetic is always indicated, esp. for an MB. I have had cars with 200k+ miles with oil pressure that was the same as when new/original, due to use of synthetic. I would recommend going to the latest MB recommended oil, per MB spec 229, which is Mobil One ESP 0-40 "M" series oil, which is specifically formulated for MB.
My dealer has the ESP 0-40 M oil in very large bulk tanks. You can learn more about it on the Mobil One site.