I suppose it depends on the amount of driving you do, especially on motorways on winter mornings with low sun. You can get through gallons of the stuff which would work out expensive. For my in-town commute each day 100% works for me.
Current: 2013 F10 520d M-Sport, Professional Media, Visibility Pack, Reversing Camera, DAB
Previous: 2011 F10 520d M-Sport, 2008 320i M-Sport Coupe, 2002 325i, 2001 318i valvetronic, 1998 318i, 1996 525i, 1990 Porsche 944S2