5w30 is recommended for cooler temperatures. If you want to thin it out a little more, you could substitute up to 20% of the motor oil with Marvel Mystery Oil. Mmo thins the oil but also helps clean the engine. All of the mechanics at my job swear by mmo for both their personal vehicles and work trucks.
Works good for motorcycles too.
1999 Alero GLS, 3400, 4T45E, current mods: WAI with Spectre HPR cone filter, VVME 55w 8000k HID kit
275,000 miles as of 05-14-2015 at 1559 est (3:59 pm)
Planned mods: F body front brake upgrade, ported UIM, LX9 top swap, ported exhaust mans, performance cam. Eventually, turbo or supercharger.
1999 Olds Bravada, 4300 Vortec v6, 4L60E. Current mods (on it when I bought it):
75mm ported throttle body, NGK stage 1 plugs, stage 1 heads.
Planned mods: injector upgrade, exhaust headers