According to 2005 Chevy Impala Owner’s Manual, Engine Oil with filter capacities are as follows:
3400 V6 Engine: 4.0 quarts (3.8 L)
3800 V6 Engine: 4.5 quarts (4.2 L)
3800 Supercharged V6 Engine: 4.5 quarts (4.2 L)
SAE 5W-30 is best for your vehicle. These numbers on an oil container show its viscosity, or thickness. Do not use other viscosity oils such as SAE 20W-50.
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 your engine at extremely low temperatures.
Tip! Learn everything about engine oil and which oil is proper for your car, truck, SUV or crossover at Chevrolet Owner Center.