Word on the “street” is that Mobil1 and castrol were sued for advertising that their oils are 100% synthetic. Not sure who sued. As a result all their “synthetic oils” are now labeled as synthetic blends. That is a fact. Its not that they are no longer synthetic it is just that they were forced to put what’s really in the bottle on the label. Im inclined to believe that that is true due to the fact that mobil1 and castrol are like 30 bucks a gallon as to where a real full synthetic oil is closer to 50 bucks a gallon. Not to mention that labeling.
while formulas are secret, I’m sure if Mobil one was really still using group 4 oils as their base they would say it instead of this BS answer:
http://www.mobiloil.com/USA-English/MotorOil/Car_Care/AskMobil/Group_3_Base_Oils.aspx
the latest formulations of rotella T6 are group 4 which is what you will find in amsoil, redline and other premiums.
I spoke with a chemical engineer who worked at Mobil for years, he said the Mobil 1 started out a pure synthetic and changed to a conventional stock based synthetic to save cost. I guess it’s legal to be marketed as full synthetic because of the amount of additives that are added during the process.
Pennzoil ultra or platinum user here. Switched after spending WAY too much time reading the whole mobile 1 debate on bobistheoilguy.com. I honestly don’t care if there is something better now, or cheaper. I wasted too much of my life last time reading oil debates so now I’m just sticking with what I’ve been using.