Answers

The owner's manual says to use 5W30 oil. It doesn't require that synthetic oil be used.

On page 315 of the manual, it says:

"If you are in an area of extreme cold, where the temperature falls below -20°F (-29°C), it is recommended that you use either an SAE 5W-30 synthetic oil or an SAE 0W-30 oil. Both will provide easier cold starting and better protection for the engine at extremely low temperatures."

So, unless you live in Fairbanks, you should use SAE 5W30 oil and it doesn't matter to GM whether you use regular or synthetic oil.

Other viscosities aren't ok per the owner's manual, but only 20W50 is specifically mentioned as an example of a weight not to use. Both 5W30 and 10W30 are 30 weight oils so 10W may be ok in your situation - I suspect GM says 5W (the W stands for "winter") for easier flow in colder temps for gas mileage reasons.

Source: http://www.gm.com/gmownercenter/

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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.

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One thing I noticed "missing" in the owner's manual was a comment telling you to look for a "SM" and "GF-4" or similar designation. GM just says to use engine oil which meets GM Standard GM6094M and displays the American Petroleum Institute Certified for Gasoline Engines starburst symbol.

I guess GM figures that the label changes now and then as oil keeps getting better and they expect the ratings to keep changing.



And it's easier for GM to change their GM6094 standard that try to keep up with the API in the written manuals.

Source: 

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Commenting on that last post, remember that manuals are a snapshot in time, and future updated oil standards are always better than the last and compatible. We are about to go to GF-5, for example, and it will be fully compatible with the older GF-4 standard.

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