I don't think there is any statistically significant difference between the failure rates on the V8 vs I6 models. The transmissions are completely different (ZF vs GM) as well, suggesting that failures perhaps aren't solely due to the designs or manufacturing processes of the transmissions.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue