The correct stock diff fluid in the E90 is Castrol SAF-XJ and its s 75w-140 wieght. I substituted mine with Redline 75w-140 and the exact quatityis 1.2 liters
Alot of the earlier bmw models run of Castrol SAF-XO wich is a 75w-90 weight.
From the Reddline website
SAF-XO & SAFX-LS => 75W90
SAF-XJ => 75W140
Im sure either one would work though.