To say Pennzoil Platinum 5w30 is "extremely weak" would not be accurate.
SAE viscosity grades are a range of values, to which the Kinetic Viscosity at 100*C is often referenced. The KV100 range for a 5w30 is from 9.3 to
So, it's is a marginally thinner than other 5w30 oils, where some may carry a KV100 of around 11.0-11.5 like Supertech or Quaker State Full Synthetic, but not so thin that it should be considered a 5w-20.
But, this does not necessarily mean that Pennzoil Platinum is inherently weak. It consists of high-grade GTL base stocks and does not use as much Viscosity Index Improvers, which means that it can be more resistant to shearing over use compared to other 5w30's.
Pennzoil Platinum 5w30 is still licensed for Dexos1 Gen2, and is formulated to "meet or exceed the requirements" for API SP, ILSAC GF-6A, Chrysler, Ford, and Honda's oil requirements.
It's a great oil and if you're concerned about how it interacts with your engine and use-case, a few used oil analysis wouldn't hurt to establish a trend to make sure this oil works for how you use it. I wouldn't run out and drain your pan to fill it with something else.
I am most likely going to use Pennzoil Platinum 5w30 the next time I change oil in my Mustang GT.