I suspect that the old gray bottle and black label Pennzoil 0W-40 SRT are the same stuff...
I also think the Pennzoil 0W-40 and 5W-40 Euro are the same add pack, just a slight change in viscosity, and that the Ultra is the same as the silver label...
Several other brands offer a 0W-40 and 5W-40 version of their oils, with the 0W-40 being hard to find, Shell Helix, Quaker State, Valvoline Synpower... Same stuff, different bottle / brand?
It would make sense, you could turn in the base oil for manufacturers approval, (MB 229.5 for example), then sell it under many names...