I've heard the same thing from multiple people regarding sticking with one brand of oil. Well, I proved it to be false. I have used 4 brands of oil in my Sprinter interchangeably depending on what was cheapest at the time I bought it. Here they are: Mobil Delvac, Shell Rotella, Chevron Delo and (gasp!!!!) Supertech (all of them dino oils, not synthetic, that carry the spec recommended by Mercedes.) Happy to say that after 729,000 miles nothing of the kind mentioned above affected my engine. To make it even worse, sometimes I even mix half a gallon of another brand at an oil change. Let me explain. An oil change takes 2.5 gals of oil, so I buy 3 gallons of whatever is cheapest, and I save the remaining half gallon for the next oil change. Next oil change I only buy 2 gallons of whatever oil happens to be cheapest then, and use the remaining half a gallon from last change. My engine doesn't know the difference. Runs as good on Mobil as on Supertech.For me, this is a no brainer. The engine comes with Pennzoil Platinum 5-20 full synthetic from the factory. If you get an oil change at a dealer, that's what they use. Walmart has it for $10 cheaper than the parts store chains - it's $26 for a 5qt jug, and you need 5.5 qts.
Technically, whatever meets the Chrysler MS spec is okay to use, but I have heard that it's important to stick with one brand of oil for the life of the vehicle if possible. As a lubricant, oil is pretty much oil, but each brand has different additive packages and I have heard that some are not compatible with others. I have heard that some have more or less or different seal conditioners, such that you switch oil brands and suddenly you are chasing oil leaks. I can't confirm this, but I'm at 123k in 13 months and I haven't had any problems whatsoever, nor am I burning a single drop of oil.
Even dimestore off brand oil is going to cost $5/qt. There's a Walmart on every corner and they are open 24 hours. I just don't see the logic in using anything else but what it came with.