I'd be interested to know where you came across this info
I havn't tried a different grade in the BMW yet but have been considering it, that's why I'm interested in any data points.
I know some Guzzi owners use a 75/140 oil in the old 5 speed boxes in preference to the recommended 80/90 recommended, stating it assisted in making gear changes less noisy and improved the gearchange.
I don't know much about gear oils though but I do know that the viscosites quoted are different from engine oils.
I've seen that a couple of folks on here run Redline Shockproof Heavy. I've used it in some Guzzis and found it was an improvement for a while but gradually degraded. I moved away from it as Redline don't produce a data sheet for it and won't give you any info, nor does it meet recognized approvals. That said there are many folks using it, Harley owners love it and many Guzzi owners use it too and I've not heard of any failures. It's incredibly sticky looking and viscous, to the point of being a PIA to drain.
I'm not recommending it, just pointing it out. Of course running a different grade to recommending could get you into trouble with any warranty defects claims as pointed out
Temperature changes in the gearbox will also be much lower than the engine, just a guess but I'd think gearbox temps would be around 60-70C where an engine would be closer to 100C.