I have an '04 Chevy Cavalier, Ecotec 2.2L. The owner's manual and oil cap say use 5W-30; owner's manual says it's ok to use 10W30 in the summer, or 0W30 when it's extremely cold, but says 5W-30 is best for the engine. Ok, so I've never been one to question the manufacturer's recommendation as far as weight goes. However, the only GC I can find locally is 0W-30. 1) Am I better off using the GC 0W-30 or a non-GC but nevertheless good 5W-30? 2) What are the consequences of using 10W30 or 0W30 in my engine right before the onset of an Oklahoma (read: hot) summer?