They make a few lines. In fact, at some of our stores, you can find up to four varieties of Castrol 15w40. I haven't seen their E6 variety out there yet, but I haven't really looked. Up here, it's probably available. Wakefield pushes pretty hard. At one store, a few years back, I saw a Castrol Elixion, a Vection, and two varieties of GTX 15w40 HDEO, so they're there, if you go to the right place.
The Rotella here would have been available through one of the distributors and our NAPA stores have it on their site, too. I think it's a good choice for me, for free or for pay. It exceeds the requirements for my vehicle, and up here, HDEOs can be found at a good price more readily than PCMOs. That's not to say I can't find a good deal on PCMO, but the everyday price on HDEOs at the distributors makes them very attractive here.