Dennis,
I'm actually running the North American Pennzoil Platinum 5W-30. I bought a 6-pack of it back a while ago at my local Checker's when I could purchase it for $0.99/quart after rebate. I figured I'd try it in the STi to see if I notice any difference in seat-of-the-pants "feel", and also see how a UOA looks in comparison to GC.
The oil does "feel" thinner to me, as the engine seems a bit more willing to rev. It also seems to make the engine slightly louder, although that is a bit hard to judge as the STi is a very noisy car between tire whine, transmission whine, etc. At this point in time I'm planning on running the oil for 5,000 miles and then drain it and send it off to Blackstone. But I may decide to drain it early if I notice anything odd with the oil (like increased consumption).
-Andy