Castrol is a decent oil, I used it long ago. I switched my first 'Wing in '96 to Mobil 1 15/50. Fuel mileage increased by .8 MPG, not much, but the transmission shifted so much easier. It just felt like a really slick oil. So, I've stuck with it since, in everything from the g'son's Elite 80cc scooter, Spirit 1100 & Valkyrie. It's an easy to find oil so if I'm away from my home dealer, no problem.
I've not used other diesel oil synthetics,(Rotella T6) but believe they would likely be as good as Mobil 1. I don't use exotics... Royal Purple, Amsoil, etc. If I can't buy it in WalMart, K-Mart, or the local corner parts dealer, it's too rare for me.
I run the Amsoil 10-40. It's pricey at $10 or so a qt but 3.5 qts lasts me a full year. I didn't get that much riding in just 5000mi last season. So for $35 plus the filter, I'm okay with that. I've used T6 lots as well but found it sheared down rather quickly. The Rotella 15-40 is probably your best bet with respect to price, lubability and easy to buy at Walmart.