Lots of people throwing around recommendations, but the ONLY oil I would run in my 2001 E320 would be one that has demonstrated it meets the proper MB spec.

All of them may be found on the list at this link:

Mercedes-Benz Betriebsstoff-Vorschriften: Motorl, Getriebel, ATF, Khlmittel, Bremsflssigkeiten

If you're in the states and don't want to mail order, you'll have to work pretty hard to find spec oils beyond Mobil1 0W40 (regular or turbo diesel). My GF's 2001 CL55 has been running the Total Quartz 5W40 since her last change and it's been great. I ordered it from that oilchangekits place.

Side note: there are other flavors of Mobil1 (and Mobil Super) now on the list, too (see snip below, I couldn't get it to embed larger so you'll have to click it). The turbo diesel one I see pretty regularly on store shelves, though I've never heard of Mobil1 New Life until today, much less seen it.

And no, I'm not a stick in the mud about oil. However, I do generally concede that my research and engineering are lacking as compared with the folks who designed the engine and developed the oil specifications, and I'm not one to choose an oil based on a slick marketing campaign, either, so I know I'm good if I stick with an oil that is on the approved list (and bears the spec on the label).

Good luck, all.