To say Pennzoil Platinum 5w-30 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 5w-30 is from 9.3 to <12.5 cSt. From what we have seen, Pennzoil Platinum's KV100 is right around 9.9 cSt fresh out of the bottle.
So, it's is a marginally thinner than other 5w-30 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 5w-30's.
PP 5w-30 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 UOA's 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 PP 5w-30 the next time I change oil in my Mustang GT.