changing to a thicker oil means the tranny will suffer. Plus, to change to a thicker oil to render the helical gearing in the torsen useless is like putting a hole in the footwell to stomp on the left front tire if you like to turn left. OK, bad analogy ;P

It will take a lot of viscosity change to render a gear useless. To the order of days not seconds (Viscosity is a number in seconds - the time it takes to empty a tube). To the fluidity of Goop.
Only viscous coupled units will have direct effects from using thicker oils. Thicker oil on mechanical gears will just make them harder to spin because of surface friction, and that includes the shaft spinning the whole differential from the housing.

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Mk1 A4 - by BSK
Mk1 TT - by QuietsTTorm