Filters don't usually cause slipping, but the bad filter could have caused the slow engagement you were experiencing.

Correct fluid has the right friction modifiers, so the wrong fluid is the most likely cause of the slipping.

__________________
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