Here is more honest, non sponsored either by bmw, audi or mb test...
Same type of cars with similar weight and size,power, fuel
Not mixed petrol w diesel.. like someone showed above...
Had a6 avant 3.0tdi quattro. Never had a single problem in area where I live, same with s4 avant quattro
Where current 540i x drive... slip... and goes side ways driving up the hill where I live for years. When It snow I can still manage to get home. However it is noticable... it not right.
Every single day I am going thru the same road...
When I drove up the hill in snow, is more like going fine and at some point front wheels start spinning or rear. Its like "smart logic" getting lost. Delivering to soon to much power to wheels and slip...
Take a test drive at dealer when it snow and compare youself xdrive vs quattro.