First of all, Mobile 1 motor oil was never a 100% true "synthetic" motor oil.

Before the tri-synthetic formulation, the base stock in Mobile 1 motor oil was true 100% synthetic (PAO and ester, Group IV and V base stock). However the 20% or so additives used a carrier oil that is dino. I think there was a footnote on the mobile 1 bottle before. So without the additives, Mobile 1 was a true 100% synthetic oil but with the additives, it is not, technically speaking.

Then Castrol changed its Syntec synthetic oil formulation (by using Group III base stock) without reducing price. I heard the cost to get Group III hydrocracked base stock is about half of what it takes to produce the Group IV or V base stock. Mobile 1 sued Castrol about that but lost in the court. That is why Castrol is till calling its Syntec synthetic even though it uses super refined dino oil.

BTW, the Group III base stock is actually very good. It performs quite similar to the Group IV or V base stock except it has a higher pour point and a lower flash point. Some famous "synthetic" oil using the Group III base stocks are: Castrol Syntec, Chevron Delo 5W-40, Petro Canada, etc.

Now back to Mobile 1. Profit drives the way company behaves. Since Castrol can maximize its profit by using a less expensive base stock, why not Mobile 1? There comes the Tri-Synthetic Series of motor oils. Here is the product description from the Mobile Website:

The Mobil 1 Tri-Synthetic Series motor oils are manufactured from 100% synthetic base stocks. These include polyalphaolefins (PAO) and two other Mobil synthetic fluids which make up the Tri-Synthetic formula. These synthetic base fluids are pure chemicals which do not contain the impurities or waxes inherent in conventional mineral oils.

Yes, the PAO base stock is truly synthetic and is of the Group IV or V base stock. It has no dino content in it. So what are the "two other Mobil synthetic fluids"? It says "which do not contain the impurities or waxes inherent in conventional mineral oil". So what if you remove the impurities or waxes from a conventional mineral oil? That is the synthetic Mobile 1 is talking about here (one of the other two). Hydrocracking removes all these impurities or waxes from conventional mineral oil to produce a super pure Group III base stock. However, this is made from conventional mineral oil, not like the PAO stock which is truly synthetic.

Mobile further detailes these two other Mobile synthetic fluids.

The other base stock in the tri-synthetic is synthetic ester and an alkylated aromatic fluid.

The ester is still synthetic but the alkylated aromatic fluid is made from minerail oi, as far as I understand.

According to Mobile:

An alkylated aromatic is a synthetic hydrocarbon designed by Mobil with a different chemical structure than polyalphaolefins (PAOs). It is fully compatible with both the other synthetic components of Mobil 1 and with conventional motor oils.

Yes, it is a hydrocarbon.

Here is a quote from the Chevron Website:
http://www.chevron.com/prodserv/baseoils/faq_answers.shtml

A recent ruling from a respected advertising self-regulatory body decided a case on the use of the term synthetic. It found that synthetic base stock are not limited PAOs. The decision said that the key requirement for calling a base stock synthetic is that it be the result of conversion or processing of one complex mixture.

So a synthetic can be dino if it is "the result of conversion or processing of one complex mixture", i.e., hydrocracking.

Mobile 1 still have more "synthetic" base than Castrol Syntec, though.

Here is a good site about the process of hydrocracking:

http://www.fluidpowerjournal.com/07SepOct98/Articles/02/purebase.htm