Adding to this, I have read many records of the viscous coupler failing, but I can't correlate if these failures were due to lack of lubrication. From what I have been able to find, #1 early failures were due to bearings being installed backwards by Visteon, #2 was the oil in the housing evaporating or apparantly disappearing or perhaps never filled in the correct quantity in the first place. #3, leaks developing in the rear oil seal. #4 Flexing/distortion in the transfer case housing that lead to oil seal leaks.

Speculation follows, I can't find much about the later design, non VC open differentials having many failures. Perhaps the viscious coupler generated more heat, and/or put more stress in the housing that lead to lubricant evaporation and/or early seal failure. More speculation, perhaps the design of the original transfer case didn't have enough oil volume inside it to work well with the viscous coupler, leading to early failures.
I recall reading somewhere that GNK (India) manufactured the viscous couplers used in the Visteon Transfer cases. (Adds in another 3rd party sourcing for parts.)

Perhaps If one tosses and drives the x-types with the vicsous coupler versions, it adds to much heat into the system and this leads to early failures. If you take out the VC, then there is nothing to generate lots of heat inside the case. (Perhaps the Count or Thermo have validatable information on this