I started seeing this a little over fifteen years ago or so, mostly in Iowa. At that time, Castrol was having issues with their A320 developing sludge. The A320 held water real bad and if I remember correctly, the pigment would come in and out of solution and would worsen the sludge issues and cause shorter filter life. The Amsoil shed water more readily and didn’t cause sludge like the A320. Since the pressure drop was now lower through the filter, the use of 5 micron filtration son the gearbox lube system was now possible. Donaldson lead that charge. Amsoil was not compatiable with the A320,so if the system was not properly flushed, foaming and sludge could still be a huge issue. Ten years later, I do not have any verifiable facts that the gearboxes get any longer life, but it did cut down on the cost of oil due to fewer oil change outs. Castrol modified the A320and also introduced an upgrade called the X320 which solved many of their issues. Mobile also implemented XMP320, I believe that also had good qualities. None of the later oils were compatible so gearbox flushes became a huge deal when changing oil brands.
The Amsoil also appeared cleaner because optical particle counters hated the A320, but seemed to function with the Amsoil.