I believe the Motul 4100 is a synthetic. All of these oils have a healthy amount of ZDDP and detergents and a good TBN. The T5 and T6 aren't as shear-stable as the T4, and so they tend to thin out over time in some engines. For this reason I would probably stick with the 15W40 in your climate. This grade has a higher minimum spec for HTHS as well.
The Rotella line doesn't have the MB 229.1 approval, but I doubt that meeting that old standard is important. Here is the comparison of the MB 229.1 spec with a few of the more modern standards that the Rotella T4 15W40 meets. The Rotella line certainly has a more impressive list of approvals than the Motul.
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Thanks for the info.
It is semi synthetic...