[JimD]

If the car started at -20, regardless of what it initially sounded like, I wouldn't change the battery. We used to use dipstick or block heaters at that kind of temperature (so you could get oil to the engine in a reasonable amount of time) and sometimes kept the battery inside the house at night. Even synthetic 0 weight oil gets thick at -20. Even good batteries have trouble generating the cranking amps at that kind of temperature. I might check the CCA for the battery but I wouldn't just replace it. Seems like it did OK.