The best way to find the correct engine oil for a car is to check the owner's manual for the manufacturer's recommendations. This lists the oil that the manufacturer designed the engine to use.
Some manufacturers may recommend multiple oils depending on the conditions. Normal oil classifications describe the oil's viscosity, or resistance to flow, at extreme cold and warm temperatures. Cold is the first part of the description and is denoted by a "W" after the number. Manufacturers test oils at 0 degrees Fahrenheit and 212 degrees Fahrenheit. Lower numbers have a higher viscosity, which is desirable. For example, people living in warmer climates may do well with a 10W-30 oil, while those in areas with extreme cold need a 5W-30.
There are also various types of synthetic and conventional oils. Premium conventional oils are the type used in most vehicles, but some high-tech engines may need purely synthetic oils. There are also blends of conventional and synthetic oils that are known for being less volatile while still cost effective and work well for larger vehicles, such as pickup trucks and SUVs. High-mileage oil, for instance, is designed for older vehicles that have been used extensively and can smooth engine operation and extend the life of the car.