The 2002-2006 Sprinter Technical Service Manual (the one the Sprinter Techs at the dealers use) states:
"The engine manufacturer strongly recommends the use of synthetic engine oils, such as Mobil® 1 SAE 0W-40."
OK, a couple of things. One is, it is not an insignificant thing that a specific brand and weight of oil is mentioned by name in the manual. Most technical service manuals simply state something along the lines of, "Use a high quality oil," or, "Use an approved engine oil," or something along those lines. When a manual mentions a specific oil, do take note.
Second, you have to start thinking European when you own a Sprinter. With American cars and engines, when they say use 30 weight oil, most any ol' 10W-30 is gonna be fine. For example, you say your Sprinter came from the factory with Mobile 1 5W-40 as the initial fill (which may or may not be entirely accurate). You may be thinking, "OK, a really good synthetic 5W-40 and I'm good to go." And you'd be wrong. American cars have engines in them that are built to accommodate a wide range of motor oils. Not so with European engines. They are so tightly spec'd to the oil that they build the engine and then they engineer an oil to use in it.
The owner's manual will list all the approved oils. Stick with the list. Always stick with the list, especially with European engines. Regardless of what you think, what a friend, buddy, mechanic, me, or anyone else says, the manufacturer's opinion trumps everyone else.
Stick with the list.
There is a small percentage of 2004 and 2005 Sprinters that had the initial fill done with Shell Rotella T dino oil. The rest had the initial fill done with Mobile 1 0W-40. Far as I know, the initial fill for the 2007 models is "Mobile 1 ESP Formula M 5W-40" (which is a way different animal than Mobile 1 5W-40). If you're going to switch to another oil at first oil change, then make sure it's an oil you want to use in there for the rest of its life. Engines wear differently with different oils, and the worst thing you can do is keep changing the brand of oil you use. And with many European engines, even changing the weight of synthetics around (summer or winter) can have the same effect over time. Pick an oil from the list, always stick with the list, and then stick with that oil. Forever. So pick a good one. hehe
The main consideration, at least initially, as to which oil to use is not the 0W, 5W, 10W, or whatever number, or the API classification, it's the "BM Sheet" spec number. For 2005 Sprinters, for example, fully synthetic oil that meets certain other specs carries the MB Sheet - 229.5 classification. Somewhere on the bottle of oil it should say something like "MB Approved 229.5". Also approved for models earlier than 2007 are 228.3, 228.5, several others.
For the 2007 Sprinter, I think the only approved oil is MB 229.51. In addition to things like the engines having EURO 4 or Bluetec exhaust particulate filters, the 229.51 oils are designed for longer oil change intervals.
Use the Maintenance ASSYST computer, it'll tell you when to change the oil. First oil change, it's not a bad idea to do it at 10,000 miles, then after that follow the ASSYST. You're probably gonna get 15,000 to 20,000 miles between oil changes after the initial change. (20,000 kilometers, according to Mercedes).
I change the just the oil
filter every 5000 miles, which extends my oil change intervals significantly.
Anyway, back to your original questions of what's best...
If it's on the list, it's best. They are interchangeable insofar as good, better and best. (That's not to say you can mix and match at an oil change, or use one brand at one oil change and then switch to another at the next oil change. You probably could, but it's not really advisable. Pick an oil and stick with it.)
Read this page carefully, especially the comments under each successive MB spec to see what the each spec is designed for. When you get to the bottom you'll really appreciate what the 229.51 spec means. Also pay attention to the listed oils that are [/i]not[/i] recommended. People who think "American" will be surprised.
http://www.whnet.com/4x4/oil.html
Pay little or no attention to the part where is says,
"229.5 and 229.51 engine oils must be used with fleece oil filter when used with the Mercedes Maintenance System (MY2005+) in M112, M113, M272 and M273 engines. With FSS paper filters are fine," unless you have one of those engines (you don't) and you are using the Mercedes Maintenance System (you aren't). People read that and go all goofy trying to track down fleece filters. The Maintenance ASSYST Computer in the Sprinter makes all that moot.
Just use an oil on the list. (Incidentally, there are a few oils that were just added to the list as recently as last week.) The first AMSOIL that came out with the 229.5 spec (Mobile 1 0W-40 equivalent) had it on the bottle, but they, uhm, forgot to convince Mercedes to actually approve it first. Whoops. AMSOIL now has an actual approved 229.51 (for the 2007 Sprinters) that is true-blue and genuinely approved by Mercedes. So, on the list of 229.51 oils, you'll see both:
Amsoil European Car Formula 5W-40
Mobil 1 ESP Formula M 5W-40
Flip a coin. They're both best. But if I had to pick one over the other, I'd pick the one that the initial fill was done with, and/or the one that is mentioned by name (if, indeed, it is) in the 2007 Service Tech Manual.
On the other hand, if you're inclined to switch to Amsoil, which is a great oil, the first oil change is the perfect time to do it. Both oils will give superior protection and lubrication. The only questions, really, are which costs more, and will the more expensive one give you an increase in the oil change interval sufficient to warrant the added cost? I'm a huge proponent of regular filter changes between oil changes. I went to great pains (and expense) to once and for all prove it one way or the other (as well as to very the validity of the reliability of the ASSYST Computer). I change filters every 5000 miles, and as a result I get about 17,500 miles between oil changes versus 12,500 when not changing filters between the oil changes. That's an extra 5000 miles that cannot easily be dismissed. Will Amsoil give me even more miles between oil changes? I don't know, and I'm not likely to find out as I've picked my oil and I'm stickin' with it.
But, if you plan on changing oil every 10,000 like clockwork, no exceptions, then it's a no-brainer - use the cheaper of the two.
The vast majority of engine wear that occurs between oil changes occur within the first 3000 miles after the oil has been changed. People who change their oil every 3000 miles just crack me up.