Oh dear. Well...as you will soon find out

Mobil 1 is a "true" synthetic, whereas Rotella is a "fake" synthetic.

When I got my car, I used the Rotella for the first change, and just recently on my second change, switched over to M1. M1 seems better, but the difference is pretty negligable, if you ask me. I do have peace of mind knowing that it is a better oil, though.
This cold weather doesn't really give good opportunites to make a fair comparison.

You'll be fine with either. Recent threads have noted, though, that the rise in price of Rotella kind of offsets any financial benefit from not choosing M1. I'd go for the M1.

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Dan
2005 E320 CDI - 246k
1987 300SDL TD05-16g, Herlevi pump, Elbe manifold, 2.47 LSD - 213k
Past: 1987 300D - 264k