Approved synthetic oil

we have conflicting stories on recommended oil by MB. According to information from MB, all new MB are fitted with Mobil1 synthetic. we also understand that there are other approved brands. Also, MB not only approves Mobil1, but includes with new MB their recommendatiion to use Mobil1.

Our experience is that Quaker state synrhetic was put in one of our autos (a new 320ew - first service) We have had very sad experience with Quaker state and Penzoil (same Company?) costing our clients and our company dearly. after 30,000 to 50,000 miles, sludge and carbon build up in crankcase and upper engine area were extremely excessive in several cars - even though each had received regular scheduled service.

We are curious as to why any other oil than Mobil1 is used by authorized MB dealers. are there any buletins or information on this? thanks!