You will find Castrol brand to be a very high dollar product. From some of the VOA's compared to the hype you hear on their advertising, I'd be more likely to call Castrol the snakeoil kings, than anyone. They sure do put on a good show though.
In my experience, Mobil has proven to provide quality products for a very competitive price. They also provide excellant technical service (on the industrial side anywho)
I think that you are mucho money ahead of the game to have an understanding of what lubricants are, and how they work in each application. Once this is more understood, the brand issue becomes less and less important.