Choosing the correct motor oil for an automobile depends on many factors, including the age of the vehicle, the weather conditions and climate the vehicle is in, and recommended oil viscosity. Most automobile manufacturers suggest a certain viscosity in the automobile's owner's manual.
There is typically no reason to change types of oil if a car has been using a certain type for several years. If 10W-30 has been used successfully, it is a good idea not to switch. Most modern engines require multi-viscosity oil, such as 10W-40 or 5W-30. The number before the W denotes how the oil performs in cold temperatures, and the number after the hyphen refers to higher-temperature performance. The lower the number before the W, the better it performs in colder weather. Thus, 5W-30 oil is excellent for colder temperatures. The 30 refers to how well the oil flows in warmer temperatures.
Other questions to consider when choosing oil are how many additives are in the oil and whether synthetic oil is a better option. Some mechanics claim there are fewer oil changes needed with synthetic oil, but this is not a proven fact. Lastly, an older vehicle may require a single-weight oil so that the gears stay lubricated. It is always best to ask a mechanic before suddenly switching the type of oil used in a car.