kma,
I'm not Marshall but I'll toss my two cents in for free:
There are numerous advantages to synthetics. Among them are better cold flow which reduces wear at cold starts, much improved winter starting in cold climates, better fuel economy, cooler oil and therefore engine operating temps, much better performance under extreme conditions (such as turbocharged engines), and the ability to extend the drain intervals to save money (and help the environment a little as well).
You change the oil when it's worn out. An oil analysis or two can give you the data you need to determine the maximum change interval, which is usually (but not always) limited by the soot level. Mobil-1 can suspend about 2% soot. On my 617 engines (like yours), that typically translates into 7500-10kmi. My newer 603 engines could go 15k-25kmi since the OA on those shows less soot than my 617 makes, but I change then at 10-15k anyway. Delvac-1 (which is a super-duty diesel version of Mobil-1) can suspend about 3-4% soot, allowing even longer intervals. Anyway for mostly highway driving, I'd say 7500-10k is probably safe, I wouldn't go beyond that without some OA to back it up. Or, maybe 10-12.5k on Delvac-1, again without OA. Changing at 5k with synthetic is, IMO, wasting money and pouring out perfectly good oil (and yes, there are people who do this).
The ONLY version of Mobil-1 that I would use is the 15W-50 weight, with the red cap. All of the xW-30 weights are too thin and NOT approved by Mercedes. the 0W-40 (black cap) is approved but it really is not needed unless you live at the North or South Pole. The 15W-50 has almost the same cold pourability, something like -50F! Best price is for the 5-quart jugs at Wal-Mart at ~$19, but the 15W-50 is not always available in the 5qt size for some reason (it has shown up at my local Wal-Mart recently though, which is nice.)
For the record, I use Delvac-1 but this stuff is HARD to find, you either need to mail order it, or buy from a local Mobil distributor. It also costs more - ~$21/gallon or higher. Amsoil is a good alternative. Avoid any Castrol, Shell/Rotella, or Chevron "synthetics" which are really just processed dino oils, not true synthetics, and aren't suitable (IMO) for extended drains. Whew. Wonder what Marshall will add.
