I run Mobil1 15-50 in the Lumina - plenty of protection, and flows great down to -47 degrees F
FWIW - I made the switch to Mobil1 when the car had about 120k miles because I was taking it to school (one hour north of Pittsburg) and I knew the car would see some really low temperature out there. Synthetic flows much better than conventional when it's cold, so the Mobil1 provided much more protection on startup. Car has 183k miles on it now, and the motor still runs great.
- Justin