Looking for a heavy, fully synthetic oil, I discovered that Valvoline's Synpower 20w-50 meets BMW's (and other premium car makes) longevity and viscocity specifications. Thia also indicates that the oil is comprised of Group IV oils, which are of higher quality than the Group III oils found in most other "Fully Synthetic" motor oils.
I drive an e34 BMW 535i. The M30 motor requires a heavy oil since it runs fairly hot for a naturally aspirated engine. The fully mechanical drivetrain also relies on a thick oil in order to properly lubricate the valve eccentrics and camshaft lobes.
This oil holds up extremely well. I had previously used Mobil 1 15w-50 oil and found that my valves became noisier as the miles racked up. After switching to the Valvoline, my lifter noise is much more quiet. The engine runs very smoothly and I'm very pleased so far with this oil'd performance.