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Just search diff noises...that will give you your answer on why BMW loaded the the OE oil with FM.
I know Mobil 1 75w-140 doesn't have a lot of FM in it because I get groaning noises at low speeds.
As far as a performance upgrade...I would go with a clutch type diff before I swapped in shorter gears. If you've got good throttle control...driving is a lot more fun. You get to feel the connection between the throttle and the rear axle. It sounds stupid but it's like a good seat...it just enhances the feel, communication and fun!
As far as speed and times...I think there's an easy 1-2 seconds per lap that's on the table in the HPDE environment...that's my guess.
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2018 F30 320iX Melbourne Red
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red
2000 E36/7 Z3 Steel Blue
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