You can get a pretty good idea about the basestock of the oils by looking at pour points and flash points. Also, many MSDS's will list the main ingredients of the basestock and content percentages. The CAS numbers (chemical abstract numbers) that are listed as 64xxx are mineral oils and numbers listed as 68xxx are the PAO's and esters.
The benefits of one group over another is more difficult to define. One of the main concerns with oils that are group 3 basestock is that the companies charge you group 4 prices. This is also a ethics issues with regard to the deception that is taking place in the labeling of the product. The group 4 oils will outperform the group 3 oils at the extreme limits with regards to flash and pour point. Oils with a strong amount of esters (gp 5) will do a better job of keeping your engine clean.
It all boils down to how well balanced the basestock and additive components work together. For my money, if I'm in the market for a synthetic oil I don't want to pay inflated prices for a highly refined mineral oil. Maybe my engine won't know the difference but I will.
[ December 02, 2003, 06:49 AM: Message edited by: FowVay ]