The suggestion to use Mobil 1 is merely a marketing deal Chrysler signed with Mobil. In exchange for Chrysler buying Mobil 1 for the initial fill, Mobil agrees to share sponsorships on certain events with Chrysler and Mobil makes a payment to Chrysler in consideration for plugging Mobil 1 as reccommended for the car. Oil companies compete for these deals with manufacturers, though auto makers rarely switch sides.

When it comes to technical standards and not marketing deals, any synthetic that meets either the MB 229.3 or 229.5 standard is acceptable for use in an SRT-6. These are fairly strict standards, and many 5W-40 synthetics DO NOT meet this standard.

Rotella 5W-40 synthetic is a great oil, and I ran it for years in my Jetta TDI, but it DOES NOT MEET MB 229.3 nor does it meet MB229.5, so it hasn't proven that it is good enough to meet the Mercedes technical standard.

The fall back position in the service manual is that any 0W-40 or 5W-40 synthetic that meets API SL or GF-3 are acceptable substitutions.

Rotella T does meet API SL, according to the Rotella T home page. <----- Link

This suggests to me that it is an acceptable fill for the SRT-6. Given how inexpensive the Rotella synthetic is at Walmart it represents a substantial potential for savings on the cost of oil.

Bonus info: There's a good list of oils that do meet the MB 229.3 and/or 229.5 standard on whnet.com. <---- Link

The linked page also lists a few oils that do meet the 229.3 or 229.5, but interestingly they are NOT approved for use in AMG motors. I wonder what the SLK32 service manual has to say about lubrication?