Rotella T is not a true synthetic.
A couple of years ago, a change in the regulations made it leagal for oil makers to advertise an oil as a full synthetic, even if there were only a small amount of actual synthetic addatives in the oil.
Mobil 1 is the only true name brand oil (Red Line, Amsoil amd Bardahls are also true synthetics but less widely available and much more expensive!).
Rotella is also NOT on the list of oils approved for use by Mercedes after extensive testing.
Here's the list: