The recommended oil weight for your vehicle can be found by checking the owner's manual or inspecting the oil cap underneath the hood of your car. Oil weight is determined by weather conditions. Any oil you put in your car should match the manufacturer's standards and be API-certified.
The most common types of oil thickness used in cars include 5W-30, 10W-30, 5W-20, 0W-30 and 5W-40. Oil's thickness is determined through hot and cold testing. The first number indicates how thick the oil is when cold. Thinner oil is better during the colder months of the year since it doesn't put as much strain on the engine and the car is easier to start up. The second number indicates how thick the oil is when hot, which also helps to protect the engine.
You may also decide to use man-made synthetic oils, which are better able to handle extreme temperatures compared to natural oils. The reason natural oils degrade faster than synthetic oils is that natural oils contain impurities that remain from the refining process. Synthetic oils are more expensive and should be changed after the same number of miles as natural oil. Select new cars may contain synthetic oil from the factory.
Use the recommended oil thickness to keep from voiding your vehicle's warranty and damaging your vehicle. Any synthetic oil you use should be recommended by your car's manufacturer.