I use Mobil1 filters on both cars.

RX-7
Summer: 20W-50
Winter: 10W-40

Civic
Summer: 10W-30
Winter: 5W-30

IMHO, 20W-50 is way to thick for a piston engine in most environments. My local Acura dealer used 5W-30 year 'round in my old Integra, when I still had that (it was dealer maintained all of it's 167,167.7 mile life... freaky number to die on)