As a guiding principle, use the lowest viscosity spread that gives you sufficient cold start (5) and hot weather (30 or 40) performance.

5w-30 has always worked fine for me, down to -30 c. If I was towing a heavy load in desert conditions I would look for a 5w-40.

Instructions for resetting the light are posted here, you can do it yourself.

__________________
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