For myself, I do not think that the deformation of the casing is the first cause of the noise, the first cause of the noise would be poor quality machining of the teeth, having dismantled and reassembled the bevel box three times, two mine and that of a friend each time before reassembly I put marker on the teeth of the spiral crown to see the traces of the wear of the input pinion I realized the traces on the crown wander while going completely around the crown toothed, I am used to rebuilding or building 901/911/914/915 gearboxes for which some have crankcases in poor condition and conical torques which have hundreds of thousands of kilometers, the problems with noisy gearboxes are so do not change the damaged bearings and if the distance of the conical torque is incorrectly adjusted or a poorly manufactured adaptable spiral conical torque is installed.
For me I use 85w140 I tried the 90 which is fine too, the Redline 75w250 is cat pee for me, the thicker oil you put the more it helps to heat the bevel box that said more it heats up the more it liquefies.
In winter I don't pay attention to the noise, I always have the impression of driving in a vehicle from before the Second World War and, as the old mechanical repairmen say, if it sings, it's happy.
Lapping the teeth and super-finish polishing can certainly improve certainly improve the sound level, the teeth of my bevel box.
After replacing only twice the cracked crankcase without the gears which have more than 100,000km everything is fine.
