"Am I reading correctly that Mobil1 0W-40 is the only oil approved for use in AMG engines?"

Ringmaster, it looks that way. They are also very specific in the document about US vehicles using specific oils. Model years. It also lists different oils for vehicles in other countries.
Stan Here is a direct link to the site that has the document, no need to download the pdf. http://www.whnet.com/4x4/oil.html