I think there is more to consider in the Mobil vs Castrol marketing question.
As Dr. Gresham, a contributor to Tribology and Lubrication Technology has stated, Group I, II and III are simply more sophisticated levels the refining of crude oils and I tend to agree.
In addition, he says that "synthetic base oils are generally specific polymers produced from controlled polymerization. These polermerization reactions begin with specific small pieces called monomers and start from very basic building blocks like ethylene, propylene, butenes, their oxides and similar small molecules. The resultant base oils are polymers of carbon, hydrogen, oxygen...most commonly the polyalphaolefins, dibasic esters, polyol esters and polyalkylene glycols...they dont' have wax [and other compounds] contaminants."
Another definition of synthetic is: produced by synthesis, especially not of natural origin, made from basic building blocks not part of the resultant sythesis, and historically, this has been used as a definition for Group IV and V base oils.
As I have stated before, I think Mobil made a mistake by bringing the issue to a marketing association such as the BBB. What were they thinking? Most issues such as these are fought in the technical world of peer reviewed journals and with the army of scientists/chemists Mobil had, they should have been able to cast a great shadow of doubt over Castrol's claims.
I think the real sad issue here is that the marketing and sales community, instead of the scientific community, is defining lubricant chemistry.
Now with all that being said, we see that the highly refined Group III formulations are showing great performance results in both PCMO, gear lubes, and hydraulic oils.
And the same can be said about HOBS and other renewable lubricants.
Additive chemistry has advanced to the point where we no longer should be concerned about additive solubility or other additive interactions with either Group III OR Renewable lubricants.
Simply put, Mobil made the mistake of bringing this issue to a marketing group and lost to Castrol. Mobil is only to blame for their own, in my opinion, miscalculations.
The oils made by both companies are the result of lots of expensive research and development and will show their merit in used oil analysis results and field trials.