To choose the right motor oil for a vehicle, find out what kind of oil the car has been using, what kind of oil the owner's manual recommends and how old the vehicle is. Also, determine how worn the vehicle's engine is.
If the vehicle has been running well, then there is no reason to switch brands of motor oil. Usually, the owner's manual states what kind of oil the vehicle should use. If the vehicle is still under warranty, using something other than the recommended oil may invalidate the warranty.
The kind of climate the vehicle is used in also plays a part in determining the right motor oil. For example, multi-weight oils cover a range of temperatures. When choosing an oil, the lower the number before the "W," the better the oil works in cold weather. The age of the vehicle is an important factor because vehicles that have been running on single-weight oil have likely built-up a large amount of sludge. If a switch is suddenly made to multi-viscosity oil, the detergent in it causes problems.
Also, if a vehicle has been running on 30- or 40-single-weight oil for several years, multi-weight oil isn't thick enough to lubricate the worn engine parts.