The owner's manual for my 2004 Chev Cavalier with 2.2L Ecotec motor indicated that 5W30 oil is recommended for year round use and 10W30 is "acceptable" above a certain temperature "if" 5W30 is unavailable. Because I was wondering if it would be a benefit to shift from 5W30 to 10W30 in the summer, I asked GM in an email the reason why 5W30 is recommended all year. I asked if, due to design tolerances, it provides better protection even in summer. It seems I'm not going to get a reply. Any thoughts?