No one ever has suggested you change the oil spec for short term climatic changes, if you lived in Norway where its cold to extremely cold 8 months of the year you'd stick to a thinner grade as opposed to someone that lives in outback Australia where cold means putting on blundstones instead of thongs with your shorts.
In reality if you are changing oil at 10K intervals the difference between a 30 or 40 will make SFA difference.

