Yikes. That's unfortunate. It doesn't help that the BMW fluid doesn't lubricate very well. It feels vastly inferior to a gear oil, in favor of having the right wet clutch friction characteristics. (at least that's my bumpkin 'squeezed the oil between my fingers' impression)
I've gone against the common advice and switched mine over to redline about 20k miles ago. Will advise if my T-case explodes because of it, but it's working fine so far.
Should be able to get a used T-case for about $200.
www.car-part.com
Punch in your vin number. The car-part web portal is very good about X-referencing the appropriate makes and models.
My e91 is using one out of a 5 series that had ~80k when I got it.