The incompatibility is primarily in late model cars,
and most important in certain models of Audi, VW, and BMW which require some very special fluids which cost about $27. per litre at our local Audi dealer. Worst of all, the Audi stuff is not compatible with the BMW stuff, and vice versa, and I have kids with both who expect daddy to always be ready to service their high mileage treasures.
Apparently this all began in the 80s, when someone at ZF decided that it was necessary to totally eliminate any change in steering feel in extreme Scandinavian climates.