Interesting - great lead. While I'd be very adamant about using BMW MTF in order to get good comparability with synchro materials and whatnot, diff oil I'd be less concerned. Amsoil SVG in the 110 or 140 weight range would be my choice.
In my view 110 would be optimal because what happened was that the API reduced the viscosity range that constituted a 90wt, which is what drove manufacturers to spec 140s. 110 is the heavier half of what 90wt used to include, so I'd be comfortable with it.
Does anyone know the torque rating and other car range uses of the diff in the 135i?
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