I read about this in a "Kit Car" Magazine and it was explained that Mobil 1 TriSynthethic (Group IV) was the company that hauled Castrol into court because Castrol Synthetic went from a Group IV lubricant to a Group III lubricant utilizing a cheaper base stock but still demanding the same higher price to the consumer.
Mobil 1 challenged that Castrol oil was no longer by definition a true synthetic and that it was unlawful for them to package and sell it that way. Well they lost their case in court when it was decided that simply chemically altering a molecule was enough to be considered a synthetic. Pennzoil chemically altered the molecule that was Parrafine wax in their original organic oil and sell it as synthetic oil...which is legal by the court case. I personally use Mobil 1 TriSynthetic oil in all my vehicles. It is as I've said a Group IV lubricant and you can see for yourself it's excellent flow in cold weather by simply pouring it in cold weather or putting it in the freezer with alongside an organic oil. The organic oil will flow like syrup and the Mobil 1 will pour normal.
Also Mobil 1 is the factory fill in some of the worlds most known automobiles...and GM Corvette oil is actually Mobil 1 Synthetic.
Dodge Viper has it as factory fill too.