Enlighten me..... You've got my interest now.
I just wrote a HUGE post about this whole topic (not just Rotella) on another site. Perhaps I will transcribe it to this forum.
Here is the readers digest version:
Rotella T6 is a Group III base stock oil with some polyalphaolefin (PAO) added. Group III base stock is a mineral oil that is highly processed through a method called hydrofracking. In 1999 a court case was lost by Mobil who sued over the term "synthetic" being used inappropriately, and it was deemed that "synthetic" is a marketing term, and doesnt relate to the product production directly. Therefor, many oil manufacturers label their Group III oils, which again, come from crude oil, as synthetic or semi-synthetic, because of the amount of processing... For marketing purposes.
A group IV oil would be a PAO base oil and would be "fully synthetic", but these are scarce and hard to find. Most "synthetic oils" are actually Group III with some PAO, so some people call them Group III+.
Super simple version: Group III is made from crude oil. Group IV made in a lab.
Rotella T6 comes from Group III.
All that said, T6 is
not a bad oil. Just not what someone like me would call "Full Synthetic."