Not trying to start a riot, but Castrol is merely a bottling company with a VERY good marketing department. They do not make their own product. For the most part, they don't even have unique additive packages. Their products are by no means inferior, though.

According to most "oil guys" I've talked with, as long as you change your oil on a reasonably regular schedule, use whatever oil that makes you feel good. Or whatever brand that "speaks" to you. In my experience, and theirs, the filter is the more important place to be picky about the product.

Me personally, I use whatever my oil supplier comps me. Right now, it's Delo 400. My fresh rebuild got a quart of Lucas, 4 quarts of Delo 15W40 dynosaur and an OE Mitsu filter.

FWIW, there's a ton of info here.