Regular conventional cheaper oil is fine, as long as it has both "SM" and "GF-4" on the label. Look for those. I would use a 10w-30 only in warmer weather that is always above the freezing point of water (32 deg F). The 5w-30 flows much better in colder weather, reducing wear at start-up, and makes possible more reliable cold starts in weather below freezing. Then change the oil precisely according to how often the owner's manual says. Your GM car has the Oil Life Monitor system, so it will tell you when to change the oil, a great convenience, and it takes the guesswork out of it.
A 10w-30 is unneccesarily thick when cool or cold outside, because that "10" number in there is too high, so why even use it? A good idea is just to use an SM, GF-4, 5w-30 all year in all seasons. An even better way is below:
Even better, use Mobil 1 0w-30 synthetic year around, both summer and winter. Walmart, Autozone, Pep Boys, Checker Autoparts, Advance Autoparts, Kmart, etc., carry it. Easy to find. The top number "30" is the main number you have to worry about to stay in Chevy's specification (in addition to SM and GF-4 of course) for your Impala. The first number, in this case "0w", just means it flows better when cold, a real plus for less engine wear and more reliable cold starts. A "5w" and a "10w" will not flow as well when cold, meaning your engine experiences more wear and you might not be able to start the engine on some cold days. Therefore, a "0w-" synthetic is best for an expensive car (all new cars are expensive.....).
I've always been a Mobil 1 synthetic fan, but Valvoline Synpower 5w-30 may be a great synthetic since they are claiming they have 4 times less engine wear during the Sequence IVA test (part of the SM set of tests) than Mobil 1 5w-30. Thats a bold statement, actually making public how well it did on a Sequence IVA test, something most oil makers keep secret due to fears they will be compared with other oils! Congrats to Valvoline (Ashland Oil Co.). http://townhall-talk.edmunds.com/direct
/view/.ee9950a/7866 for more info and web links.