Hmm...I thought the 10-40 "falling apart" was a thing of the past (80s) with better additives today. People used to stick with closer multiviscosity because of the breakdown of "stretched oils" but the oils today should have better additives. Not sure about the gearbox part though.
I use Mobil Delvac (Rotella equiv)and the gear box is happy along with the engine. Shifting is so much smoother over the Rotella too.
Oil just gets better, although some will dispute that re reduced levels of "nominal zinc", for example.
So called “falling out of grade” is a simply another characteristic that has been improved over time.
My guess is that any engine oil, intended for use with an extended drain schedule, that is required and/or blessed for use by sanctioned for use BMW or Mercedes, has been proven not display any problematic level of "falling out of grade”, and that other top tier oils will behave similarly.
In other words, present day good stuff used in drain schedule consistent with the oil and its actual use, won’t display a problematic level of "falling out of grade”.
As some insight towards the concept of “falling out of grade”, is the result of a multi-grade gear oil subjected to a KRL 20 Hour Shear Stability Test. The short story on this point is that it is impossible to make a multi-grade oil that does not exhibit some loss of viscosity after being beaten to death for 20 hours, and the real issue is “by how much”, and not at all a yes or no. As long as the degradation is within the “OK” band, it simply is not an issue.