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Ls adds need only be added to prevent noise and chatter in LS diffs. The addition of LS adds in a finished product is more of a "value added" issue.
LS adds can harden the gearface as I've read, but I've not heard that gear lubes with FM adds run hotter - it should be the reverse.
AFAIK, the 1 series uses an "electronic" LSD that strategically applies the brake to a spinning wheel. It is an open diff.
BMW did have a TSB sometime back specifying XJ for all LSD models in all cars. However 90wt or XJ (which is 140) are acceptable for open diffs, as I recall.
The funny thing with 90 wt lubes is this: every viscosity in the motor oil and gear oil system is actually a range of viscosities (measured as centistokes at 100C.). A few years back, API split up the 90wt gear oil range such that the lighter (lower visc) part retained the naming of 90, whereas the upper half of the range, which had butted up against the 140 range was relabeled as "110". So you can get a 75w-110, though they are not readily available just anywhere... IMO BMW made the XJ mandate to prevent overly light 90wt fluids from being used in diffs where a lot of power was to be transmitted (i.e. LSDs on the M cars) and film thickness may not be sufficient under heavy load.
I'll need to review all thelinks above, but if a 90wt was recommended for a 135, I'd be careful in selection for as viscous of one as possible or a 110 or XJ, given the ready availability of torque at such low speeds.
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