[Netwon]

Remember the 0W or 5W only matters on start-up performance. Which is a flow/pressure rating @ -35 and -30C respectively. So in the temperatures encountered in warmer climates the measurable difference is small.

With the above in mind I am sure dealers use which ever oil suits the most cars they have in that area and climate not what is specifically best for the M5.

I would think 0W-40 would be the ideal choice. When cold pours no slower than a 0 weight. When hot doesn't thin out more then a 40 weight. Really the best of both worlds.