Mobil 1 is primarily a group III base stock. It's not a blend, it's actually a hydrocracked petroleum base stock or "highly refined petroleum". They refine group II dino to remove many of the impurities, creating group III.
Actually, Pennzoil IS a group III base stock, and was openly admitted by Pennzoil themselves. The quote below os from their
Q&A from Bobistheoilguy forums.Platinum I'm not 100% sure about, but I'm convinced it's also a group III. The reason is, Pennzoil is more focus on "factory clean" and not base stock long term performance. Since there group III base stock has a NOACK of 10-12%, it will still pass the API SN certifications that will allow a NOACK no higher than 15%.
The only lubricant you can grab off the local parts store shelf that is guaranteed to be a group IV, is Royal Purple. If you want to order through the internet, choose either Red Line or Amsoil.
Be careful though, Amsoil OE and XL lines are group III base stocks. Their signature series and high ZDDP formulations are group IV.