For dino oil, the BEST stuff on the market is Delvac-1300 and Delo-400. For synthetic, use Mobil, Amsoil, or RedLine diesel-rated products in xW-40 or xW-50 weights (not any xW-30 or thinner). There are some other exotic synthetics that are also great, but not commonly available. The Mobil stuff is excellent and easy to buy nationwide.
For officially approved oils, there are Mercedes spec sheets (numbered "228.5", etc) that list officially approved oils. Mobil-1 15W-50 is on the diesel approved sheet and is what I recommend for most applications, plus you can extend drain intervals to 7.5-10kmi to help offset the extra cost...
