I would pony up for the MB 75w85. The Mobil 1 does not have an MB spec.
You will make the money back on better fuel mileage being that is an 85 and not 90 weight oil.
Part #001 989 3303 12
If MB uses 75W85 or 85W90, I think that using Mobil 75W90 covers both MB products. May be Mobil1 Gear Oil is not approved for now by MB, but will be in a future. Every time when I open the hood, I see label "MB APPROVES MOBIL1". Mobil1 engine and transmission oils are been approved for long by MB and it is going to be a matter of time that gear oil label to say, "Meets MB 235.7 Spec". By the way, some other company make MB's oils for them, which they sell to people not thinking enough and willing to pay extra for MB brand oil or grease. Every car manufacturing company does this, they will tell you that you have to use their products otherwise you may have problems - it is about profit. Why MB does not make cars like the ones from 70 and early 80, because once you buy they do not see you except for oils and tires, if you go back to them. That is why over 75% of the cabs in Germany were MB E class, not anymore since the stupid merge w/Chrysler group may be in the future this will change again. My dad was a truck driver. He was saying that MB trucks are like a mule - they keep on going with less maintenance forever and you still can see many of old ones from 50 to late 70 running in Africa and Asia. Even the new ones are that way, but not the cars. Why do you thing they change engine oil on those trucks every 90 000 - 100 000 miles (not kidding look the attachment), but on the cars is 10 000 miles - PROFIT, that is why. MB cat is not average JOE's car that why they expect most of the owners to go back and make profit from them, but if you buy truck for business is a different story, and they want you to be happy, otherwise you will buy something else next time. Statistically 90% of the people who own MB go and buy MB next time again, because MB is a name associated with prestige and you want to keep the image you build.
At the end, it is your money and you spend them the way you want. Do not say to someone "do not use this oil, or go and dump the oil which you just putt, because the label does not say "APPROVED BY MB". Look at the facts, thing and then make decision, INFORMED DECISION, as we say in the medical field. Gave the people something that they can see, do not just say MB approval# or part# then they can chose what to do. Those forums are to help people when they have problems, or want to know something in advance, instead I see too much TALK w/o saying anything. I PM to couple of members asking something and guess, they never answer. I think that they run in some problems that I try to avoid asking upfront, but they will not share.