I suspect you are correct. I think I read where Mobil changed their base stock because of some loop hole that was opened when some consumer laws were changed that allow a "Synthetic Blend" to be called synthetic. I guess the word blend has to appear somewhere on the container. I wonder how many people get home and realize that "the bargain" they just got on synthetic oil is actually just a blend. I am curious as to how much synthetic has to be in it to call it a blend.
I've read elsewhere that semi-syns are usually 80dino/20syn