If you google "rotella full synthetic 5w-40, look for the link that is connected to www.mercedesshop.com It talks about how Rotella T Synthetic is made with Shell's XHVI base oil, which unlike other group 3 base oils does not begin with distilled crude oil, hence of all group 3 base oils, XHVI is truly the only one that can legitimately be called synthetic. XHVI is a wax isomerate, meaning that it is made from the slack wax removed from distilled crude in normal solvent dewaxing. This slack wax is catalytically transformed (isomerized) and hydrofinished into a chemically pure base oil which rivals PAO in virtually every catagory. There are other "synthetic" oils out there based on group 3, but Shell's is unique in that it uses XHVI base oil. (And I beleive Castrol uses Shell's XHVI too.) Cheveron and Petro-Canada produce group 3 base oils that come close to XHVI, but even though these oils are all hydroprocessed and utilized the same type of isomerization technology employed in the making of XHVI, they are not the same thing. Only XHVI is made from pure petroleum slack wax and its CAS number is 92026-09-4. The CAS number for the more typical all-hydroprocessed group 3 base oils is 64742-54-7. If you want to know your synthetic oil is made from, take a look at the MSDS and look for these numbers. (The CAS number for PAO is 68037-01-4.) Interesting isn't it! Feedback please!