All BMW LongLife-approved oils are xw-30 or xw-40. The BMW-labeled oil that they sell at the dealers for use in almost all BMWs (Castrol TXT Softec in disguise) is 5w-30, albeit a very thick 5w-30.
They do seem to like thick-ish oils, to a point of course. That doesn't preclude the possibility of a slightly thinner oil with superior chemistry working well.
As far as viscosity, 5w versus 0w only makes a difference if you live where it gets really cold. If that's not you, don't sweat it. The only thing you should always bear in mind when trying to decide between 5w and 0w is viscosity index improver content. Less is almost always better. Typically narrower viscosity ratings (e.g. 0w-30 compared to 5w-40) have less, so if you don't know, err on that side.
For BMW, the line in the sand seems to be the ACEA A3 spec. BMW owner's manuals usually say something along the lines of "if you absolutely can't get anything LL-xx certified, at least make sure it's ACEA A3 certified." As stated previously, ACEA A3 requires the oil's kinematic viscosity to be relatively stable over a long period of time under hard use, and also requires a HTHS viscosity of at least 3.5.
Amsoil TSO (Series 2000 0w-30) has slightly lower kinematic viscosity when hot than the BMW-labeled synthetic, but IMO that's not really a big deal. The most important point of contention is its HTHS viscosity of 3.2. That's not a life-and-death matter, but it's significant. Whether that actually translates to reduced wear protection depends on a lot of things about the oil's chemistry, e.g. film strength, which theoretically could be higher despite the oil's lower viscosity.
I suspect TSO probably would be just fine in most BMWs. The reason you don't hear a lot of people talking about it is twofold:
- The people who don't know or don't care about actual viscosity ratings (beyond what's on the front of the bottle) usually don't see why they should spend nearly $8/qt for oil.
- The people who will spend that kind of money for oil have access to a variety of other options with higher viscosities, so they don't see why they should take the chance on an oil with thinner viscosity ratings.