I've never replaced a fuel cap. I have had the light come on three or four times, and simply undoing and retightening the cap has worked every time. It usually takes a few days for the light to go out on its own, in my experience.

__________________
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