In theory, a 0w would be better but there are a couple of reasons I tend not to recommend them. For a start, there aren't many and they are generally fairly expensive. The other reason is that a lot of 0w-40s are quite thin for an SAE 40 (it makes it easier to make as 0w to 40 is a large viscosity gap to cover, so making it close to a 5w-30 makes it easier and cheaper to make). That can lead to increased oil consumption.
A problem with 5w-50 and 5w-60 oils is that because the viscosity gap is so large, a lot of long chain additives are needed. They can then be cut by the shearing action of the engine, and the oil can end up as a 10w-40/15w-40/15w-50 etc pretty quickly rather than what you want it to be. The less additives in an oil, the more stable it is.
Generally, we stock the oils that we want to sell, there are a couple of other decent oils that we don't deal with, but there are plenty of others that are overpriced/not very good/don't meet manufacturer specs etc etc etc. I can't really say why some of the other manufacturers oils aren't what you want as a few of them get a bit upset when I give my opinion, but I can justify using the oils that we offer.