Oil

With modern multi-viscosity oils you can use the same grade oil year round in most climates. Check your owner's manual for those extreme exceptions. So go with the number stamped on your oil fill cap which will probably be 5w-20 or 5w30 weight. I think it's wise to use a synthetic which costs only $10 more a change if you do it yourself.

Unless you live in extreme cold use the same oil as always. Your owners manual will have a chart showing the correct oil for different temperature ranges.

The car should run 5w-30. NEVER run a thinner oil than that or engine damage WILL occur

My manual states that 5W-30 is the overall recommended oil. In extremely colder temps, 0W-30 or 5W-30 (both full synthetic) should be used. The pre-Dexos oil spec is GM6094M, which any of the above weights as well as any name brand 10W-30. As for the 5W-20, I doubt it will cause a problem in a well cared for engine in cooler temps. But some say a XW-30 is necessary to maintain proper pressure for the timing chain...