First, a few questions for you:
What year Mercedes?
What type of oil does the owners manual specify?
What are your driving conditions?
How often do you wish to change it?
What is your definition of "best oil"?
I use Mobil 1 15w-50 in my 560SEC. Look in your owners manual and the oil weights listed tend to be on the "thick" side. I believe the reason is because its a 2 valve push rod design. Dealership uses or use to, 15w-40 here in Virginia.
Come wednesday, the dealership is doing the 45,000 mile checkup and changing mine with 5quarts of 15w-50 SuperSyn and 3 quarts of 0w-40 of similar vintage, then after a relatively short run of around a thousand miles I'm changing to all 15w-50 M1EP. This car has been parked for six years. Still perfect and runs great, M1 since 4600 miles in 1989 and all stamps accounted for in the maintinance booklet. Total miles current 43591.
If mine was a daily driver I would use 0w/5w-40 in the winter and 15w-50 in the summer, of any top tier brand.
Do you park it in a garage? then 15w-50 year round.
I used to have the same car, though it was an '86. A wonderful battleship to drive (thus my screen name), powerful and pretty reasonable on gas considering it weighs 4000 lbs. and is powered by a V-8 designed in the '70s. I ran it to 197,000 before it was totaled by a moron teen girl sideswiping it while it was parked. Much as I love my gold '97 C230, I wish I still had the big grey beast.
In Denver, CO, I used 20W-50 in summer and 10W-30 in winter, usually Valvoline. When I came back to New Orleans, which (dammit!) hardly ever cools off, I switched to Chevron Delo 15W-40.
At 180,000 miles, my oil pan developed a hairline crack and leak. When my regular mechanic replaced it, he told me, the engine internals looked as clean as the proverbial whistle.
-- Paul W.