Not in the X5, only in vehicles running in very cold climate (<-40C or F). At that temperature, steering racks tended to go solid after 10 minutes without any oil pumping through, ie highway driving. It was a real pain when you eventually did want to turn, just like the ignition lock was on. A synthetic in that application bought us a few more degrees before it happened.

What problem are you solving by moving to a synthetic? I can't imagine it is that cold where you are. A good quality synthetic shouldn't hurt it, but just not sure what difference there would be from a regular ATF/PS fluid for 99% of applications.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue