It is usually specified in the owner's manual and sometimes on the oil cap on your engine. Different viscosities are used in different climates. Most cars use either 5W-30 or 10W-30. If you live in a climate that's chilly year round, you might opt for 5W-20. I live in PA, and I use 5W-30 which is specified on my oil cap and in my manual.
5w20
SAE 5W X 20 WEIGHT
5w30
2.7L uses 5w303.5L uses 10w305.7L uses 5w20
5w30
10w30
5w30
5w30
10w30
5w30
5w30
10w30