They make a few lines. In fact, at some of our stores, you can find up to four varieties of Castrol 15w-40. 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 15w-40 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.