If it's truly stiffer in one direction, something is binding in the steering linkage or fluid pressure is unevenly applied within the steering rack. A fluid flush and new filter/reservoir housing eliminated this problem for me.

2002 X5 3.0 297,010 miles
2014 428i 15,900 miles

2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles