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My searching has helped me form a few opinions. I am not claiming to be an oil expert, but for what its worth, here they are:
The extended intervals for synthetics are no myth. The additives in synthetics are designed to last longer.
I guess "normal driving conditions" should really be labeled "easy on the engine driving conditions." But synthetics give other advantages. They don't combine with blowby and water at any where near the rate a conventional oil does. They have superior heat protection, cold flow and lubrication.
I do know of testing done by Amsoil on a fleet of NYC taxis. They went 60,000 miles between oil changes. I would say taxis do not fall into the normal driving conditions. Of course Amsoil recommends 25,000 miles between changes under "normal" conditions.
If you really want to know when to change your oil beyond any doubt, an oil analysis lab is the only way. That is the way large fleets determine when to change.
For all practical purposes I believe you can rely on BMW specified intervals. That is what I will probably do.
Maybe I will use an analysis lab. I was always curious about what they would say about my oil after 6 or 7,000 miles.
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-Bart 
2002 Bluewater/caramel
Any other car is a compromise
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