I do not think the Pennzoil Synchromesh is identical to GM's. In fact, GM does not publicize the ingredients of their formula. It must be a deep, dark secret so Pennzoil's cannot be the same. Coke / Pepsi??? I say stick with a proven winner and get GM's.
Because I'm basically a GM guy that still owns a pair of Camaros, I'll share what I know about where to get it:
I married into a GM family. My father-in-law is a 30-year GM guy. Funny how I sold my Cougar and bought my first new car, an '83 Camaro one month before I picked up my future wife for our first date at her parent's house. I can only imagine what would have happened if I picked her up in my Cougar. I did not know about her family's GM lineage at that time. Maybe we wouldn't have married????
Conversely, can you imagine what it was like showing my father-in-law my Lightning when I bought it about two years ago, then my Cobra six months ago? Get this: My brother-in-law (who still works for GM in Michigan) was visiting my in-laws here in Arizona last Christmas. I bought the Cobra while he was here. He was the first person to see it when I drove home with it. When I popped the hood, you should have seen his jaw drop.... it was cool.
BTW, my first car was a '69 Mach I and my second was a '70 Boss 302.