I do understand your frustration.

I don't think it is a differential design issue, since it hasn't been widely reported. If it applied to all vehicles in a serial number range it would be more widely reported. It sounds much more like an assembly issue or parts problem with the vent, but we need to wait and see how it is finally resolved to know for sure.

They would be disingenuous if they knew the repair wouldn't fix it, and didn't say so. It appears they found a PUMA case, tried that repair procedure in good faith, and it didn't work. They may actually need time for figure out what the real problem is and how to fix it.

Differentials have been around for years, as you say, but so have plugged differential vents, and I remember them being not limited to any one brand. I googled that phrase. The top eight hits (in order) were related to Toyota, Ford, Mercedes, Chev, GMC, Honda, Isuzu, and Dodge. They weren't all necessarily frozen, but they all had oil leaks due to a plugged vent. It can happen to any brand of vehicle.

Yes, it is frustrating. You could ask them for a loaner and to keep the car until they are sure they have fixed it. They are unlikely to want a used car back though, for something like this. Hope you get it resolved quickly.

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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