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oh joy, here we go again!
First, the only Mobil synthetics approved for use in MB diesel engines are the 0W-40 (black cap), 15W-50 (red cap), and Delvac-1 5W-40. DO NOT use any of the xW-30 weights (silver, green, or blue caps). For winter use, even the 15W-50 will be fine in sub-zero temps. Stick a quart in your freezer and see for yourself how it pours. You'll gain almost *nothing* by using the super-thin stuff like 0W-40.
Second, Castrol Syntec and Shell Rotella "Synthetic" are not true synthetics. They are modified dino oils (Group III) that exhibit some of the properties of real synthetics (Group IV/V). They are cheaper to buy for a reason - they are inferior, and will break down sooner. If you change every 3-5k, they're probably fine. I would NOT attempt extended drains with these oils. I also seriously dislike their deceptive marketing tactics and refuse to buy them.
Third, any Cx-rated oil (CG, CH, CI, etc) is technically ok to use if it's xW-40 or xW-50, AFAICT. Never ever use non Cx-rated oil that only has an Sx-rating (SJ, SL, etc). Those Sx oils can't suspend the soot produced by a diesel.
IMO, Delvac-1 is probably about the "best" diesel oil out there. It can suspend nearly 4% soot, or about double what Mobil-1 can. The only other oils I know of that can suspend that much are Delvac-1300 and Delo-400, both dino oils. Amsoil and RedLine also make top-quality synthetics, but they're a little more costly, and I honestly don't know what their soot suspension limit is (but I assume it's at least 2%).
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