The white blocks, particularly the older ones, run better on a heavier weight due to the ring pack design. It was revised in '99 to better accommodate the lighter oils, but it's still a 20 year old engine design.
It will run on a 30 weight, and in fact a 30 weight improves turbo spool up. Most dealers in the Northeast are pumping 5w30 into them (because it's cheap). It won't hurt the engine. But consumption trends up. On the older white blocks before the revision, it goes up a lot.
The big reason people lean towards the lighter weights in these is fuel economy. In a fleet/commercial context, the fuel savings adds up. But on a car driven 1,500 mi/year . . . who cares?
No way, no how, do you run a 20 in a white block. Anyone recommending a 20 just doesn't understand this particular engine. It's not a recommended weight, and no dealer or indy will ever put that into one if given a choice. Any more than you'd pour it into a BMW that needs a 60. Still don't believe me? Check the manuals, call a few dealers, and then ask a couple indys. They'll all start at a 30.
I prefer a 5w-40 in the NA because they are not speed demons to begin with, consumption falls to nil, and the engine is better protected. I still get mileage in the very high 20s. I'll lean towards a very heavy 30 in the turbos (GC for instance), but balance that against spool up.
Castrol is just a cross-marketing plug. You can run a variety of oil brands in these, and they all more or less work well if the spec and weight measure up.