My normal towing is a 1200-1300 pound PWC rig, at least 3000 miles per year. I hardly ever know it's back there. Heaviest thing I've towed is a 1970 Firebird body - probably 4000 pounds with the U-Haul auto transport trailer. Never noticed any difficulty, but I keep the transmission in "sport" mode to bump up the shift points.
I've never changed the transmission fluid either
2002 X5 3.0 304,600 miles
2014 428i 19,000 miles
2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles
1970 Firebird - Under restoration