Mercedes of Ausie should have a recommended list of oils for your engine.

What is the range of temperatures you'll get till the next oil change?

do you do a lot of high speed long distance in high temp driving?

do a search on the forum for the recommended oils approved by mercedes - may be a North American or world wide list - not sure.

how much is the Mobil1 synthetic where you are? maybe a 15W50 or 0W40