Updated info shown in RED.

Which Synthetic Oils are API Base Group III & IV?  I have contacted several of the oil companies directly, via email, to answer this question and will update this thread as I get information.  I am also trying to get a good explaination of the difference between group III vs. group IV oils.

"Royal Purple uses Group IV Base Oils."  David Canitz, Tech Services Manager (Royal Purple Ltd), dcanitz@royalpurple.com 

"The AMSOIL XL7500 series oils use the group 3 base stocks. All the other 4 cycle oils use group 4."  Jim Van, AMSOIL Technical Services 

"Thank you for contacting Red Line Oil, the synthetic base stock we use is a combination of group 4 and group 5 stocks."  Dave from Red Line Oil

Mobile 1 responed and said they are fully synthetic so I have requested specific clarification, I am nearly positive that they are group 4   

Castrol has responded with the claim of 100% synthetic base oil; however, based on the Mobil 1 vs. Castrol court case it can be assumed that they are group 3.  I have requested clarification.

Valvoline said that they have base oils that are build up (group 4?) from small atoms and others that are broken down and modified (group 3).  I have also asked for clarification from them.

Valvoline Synpower, Pennzoil Synthetic & Castrol Syntec are compared to AMSoil XL 7500 leading one to conclude that they are all Group 3 oils.
http://www.thebestsyntheticoil.com/xl_7500_1.htm

AMSoil (Group 4 not API certified) also has a comparison with Mobil 1 which are Group 4.  AMSoil created the XL 7500 series to obtain the API certification at a reasonable cost.  Mobil 1 Group 4 products are API certified per their website, a point for them.

API Base Group III & IV seems like it should be a simple question to answer, I tend to be skeptical of companies that can't, or are not willing, to provide specific product information.

Clarification of API Base Group 3 vs. Group 4

From Royal Purple:
"Group III oils are mineral oils refined from crude that have received more processing.  They are still mineral oils with contamination that is not
bothered to be refined out.  Group IV oils are man made hydrocarbons that have no contamination and are all uniform in molecular size, resulting in less internal friction and no contamination.  Contamination is what causes oils to break down, be it fuel, dirt, or water.  So if you're starting with an oil that already has contamination products still in it due to lack of refining, it will not work as well as a synthetic version."

From AMSOIL:
"Group 3 basestocks are derived from highly refined petroleum crude. They are heads and shoulders above conventional petroleum oils and because of their molecular can be legally classified as synthetic. Class 4 and 5 basestocks however, are made from laboratory made chemicals."


Here is a good link for a synthetic oil education

http://www.mkiv.com/techarticles/synthetic_oil_article/index.html

The article provides some information directly from synthetic manufactures (Redline, Amsoil, Syntec, etc) but they don't all agree on everything.  It explains how and why synthetic oil got a negative reputation and how it is different now.  It also provides some reasons why auto manufacturers still recommend 3,000 miles changes (My synopsis: Due to warranty concerns manufactures like to have the opportunity to periodically inspect the cars).  The bottom line is that synthetic oils are always superior to petroleum oils; however, under normal driving conditions the difference may not be noticeable.  Synthetics maintain their chemical properties at much more extreme temperatures (above 400 degrees and below -30 degrees), but how often are then conditions seen?  Using them can be beneficial if an engine failure is experience.  If a motor overheats, for example, a synthetic oil will provide better protection.  Is it possible that some of the differing opinions on drain interval are a result of the difference between Group III and Group IV synthetics?