Got this from "Sarge" on the LS1.com forum....

I have kept my big mouth shut for awhile until I could develop some decent "proof" of what I suspected. Mobil1 is no longer a 100% Group IV Synthetic PAO basestock. This all started post Katrina when Mobil1 announced it's PAO plant was whacked hard. Virgin Oil Analysis began popping up on various boards refelcting Group III basestocks with drops of GroupIV PAO's. Pennzoil Platinum did the same thing....went to a Group III basestock in November 2005 by the way. And I do watch many boards and forums outside of BITOG.....(which has turned into a zoo by the way).....I read the corporate reports of Exxon-Mobil and Shell.....so I research and get my info from more than the Internet and VOA's....
I wrote a letter to Mobil asking if their basestock was still 100% Group IV and got a "canned response" like others word for word by the way....
" Mobil 1 motor oils are 100% synthetic, utilizing the PAO basestock and proprietary blend of additives that is tailored specifically for each viscosity. Mobil does not discuss specifics about our motor oil formulations."

You want to know why I have an issue with that statement? Because Mobil Pre-Katrina always said and advertised they used Group IV basestock exclusively. "Utilizing the PAO basestock...." is not the same as "Exclusively using the PAO basestock..."......that is a change for sure....the other thing that has me convinced Mobil1 is now just another synthetic "blend" is they are now advertising they utilize "Synthetic Technology"....
So.....Mobil in their Corporate Report admit to disaster at the plant that made the PAO basestock ( which Amsoil buys there by the way). VOA's clearly show a reformulation with Group III showing up. Mobil has changed the way they "express themselves" now. Mobil1 now sells it for like $5-$6 bucks a quart for a Synthetic Blend oil. And that is a rip off as they are now just as guilty as those they sued and litigated against years ago.
I remain an advocate that Mobil1 is good oil. However it is very very overpriced for what it is now. Lot better oils out there for less money for sure.

Part 2 from "Sarge" on LS1.com forums...

And a little "guide book" and history for those who do not live on engineering/chemist forums for "fun"

All synthetics are not equal. The API has not come out and defined what is "synthetic", but rather, classified oils into five major groups.

Group I base oils are the least refined of all of the groups. They are usually a mix of different hydrocarbon chains with little or no uniformity. While some automotive oils use these stocks, they are generally used in less demanding applications.

Group II base oils are common in mineral based motor oils. They have fair to good performance in the areas of volatility, oxidation stability, wear prevention and flash/fire points. They have only fair performance in areas such as pour point and cold crank viscosity. Group II base stocks are what the majority of engine oils are made from. 3000 mile oil changes are the norm.

Group III base oils are subjected to the highest level of refining of all the mineral oil stocks. Although not chemically engineered, they offer improved performance in a wide range of areas as well as good molecular uniformity and stability. By definition they are considered a synthesized material and can be used in the production of synthetic and semi-synthetic lubricants. Group III is used in the vast majority of full synthetics or synthetic blends. They are superior to group I and II oils but still have limitations. Some formulations are designed for extended oil changes. Amsoil XM Oils, Castrol Syntec and many others fall into this category.

Group IV are polyalphaolefins (PAO) which are a chemically engineered synthesized basestocks. PAOs offer excellent stability, molecular uniformity and performance over a wide range of lubricating properties. Amsoil Synthetics and Mobil 1 primarily use group IV basestocks. PAO is a much more expensive basestock than the highly refined petroleum oil basestock of Group III. (Can you say profit margin! Grab your ankles and sing along!)

Group V base oils are also chemically engineered stocks that do not full into any of the categories previously mentioned. Typical examples of group V stocks are Esters, polyglycols and silicone. Redline uses an ester basestock.

In the 90s, Mobil filed suit against Castrol for falsely advertising Syntec oil as synthetic, when in fact it contained a Group III, highly hydroprocessed mineral (Dino) oil, instead of a chemically synthesized (group IV or V) basestock. Due to the amount that the mineral oil had been chemically changed, the judge decided in Castrol's favor. As a result, any oil containing this highly hydroprocessed mineral (Dino) oil (currently called Group III basestock by the American Petroleum Institute) can be marketed as a synthetic oil. Since the original synthetic basestock (polyalphaolefin or PAO) is much more expensive than the Group III basestock, most of the oil blenders switched to the Group III basestock, which significantly increased their profit margins.

hmmm...interesting info i use mobil 1 5w-30.....works for me. he he...yeah i heard about that how thier oil isnt really "fully synthetic" now is that castrol syntec stuff the same way?

Castrol ...im not too sure about, but I do know that Full synthetics are Amsoil, Royal Purple Racing 21 or 41, Cosworth, and Redline. Synthetic blends include Shaffers, and Pennzoil Platinum.