Here is a good description of what has happened as taken from one of the Amsoil numerous websites.
""Late in 1997, Castrol changed the formula of its Syntec "full synthetic motor oil", eliminating the polyalphaolefin (PAO) base stock (that's the "synthetic" part, which makes up about 70% by volume of what's in the bottle) and replacing it with a "hydroisomerized" petroleum base stock.
Mobil Oil Corporation, maker of Mobil 1, "Worlds Leading Synthetic Motor Oil," said no fair and took its complaint to the National Advertising Division (NAD) of the Council of Better Business Bureaus. NAD often arbitrates between feuding advertisers on their conflicting claims.
The notion behind synthetic motor oils as we've known them is an elegant one. Instead of relying on the cocktail of hydrocarbons contained in crude oil, why not go into the laboratory and build the perfect base stock from scratch, molecule by molecule, and builds it till it gets 10-carbon molecules, then combines three of those to form PAO. The result is a fluid more stable than the usual base oils derived from crude. It keeps flowing at low temperatures. It's more resistant to boiling off, and more resistant to oxidation, which causes thickening with prolonged exposure to high temperatures.
Still, there's more than one road to the point B of improved stability. Petroleum refiners in recent years have learned how to break apart certain undesirable molecules - wax, for example, which causes thickening of oil at low temperatures- and transform them by chemical reaction into helpful molecules. These new hydroisomerized base oils, in the view of some industry participants provided properties similar to PAO's but only cost half as much," Lubricants World reported.
The argument before NAD tiptoed around the obvious- does the consumer get four bucks' worth of value from each quart of synthetic oil?- and plunged straight into deep semantics. Mobil's experts said "synthetic" traditionally meant big molecules built up from small ones. Castrol's side held out for a looser description, defining "synthetic" as "the product of an intended chemical reaction."
What do unbiased sources say? It turns out that the Society of Automotive Engineers (SAE) and the American Petroleum Institute (API) both have technical standards covering motor oils, and both of these organizations in the '90's backed away from their old definitions of "synthetic," leaving lots of room for new interpretations.
In the end, NAD decided that the evidence constitutes a reasonable basis for the claim that Castrol Syntec, as currently formulated, is a synthetic motor oil, said Lubricants World.""
So, with the advances in the basestock technologies the performance gap in between the hydrocracked Group III and PAO based oils has significantly narrowed or even disappered.
Still the Mobil 1 fans are mostly a bunch that won't admit the defeat and continue to hate Castrol for changing the rules of the game even though they are the ones who are benefiting from the situation since there is a greater choise among the high quality oils which also helps keep the prices in check.