Did some more testing past weekend and my G20 is reliable again, it is confirmed to me now that the worn recoil spring caused the feed failures. The standard 17 lbs spring in the G20 probably becomes a 15 lbs spring after 4400 rounds and this may be too weak, especially when it's freezing.
Obviously the new spring did the trick, but it sounds like thick lube wasn't helping.
I was using Break Free CLP, but just to make sure I did switch to Remington Rem Oil with teflon for sub zero temperatures, which is a finer oil. But I don't think it was the oil though as Break Free CLP is known to work well at sub zero temperatures.