Choosing the same type of oil a car owner has been using for his vehicle and checking the manufacturer's manual to determine the suggested oil for the car are good tips for buying motor oil. Car owners who live in areas with constant temperature changes benefit from using multi-weight oils.
Owners of vehicles with worn engines that have been using 30- to 40-single-weight oils should buy heavier single-weight oils instead of multi-weight oils because multi-weight oils do not effectively lubricate worn-out engine components. If a car is under warranty, it's best to avoid using oils that are not recommended by the manufacturer to keep the warranty valid. The manufacturer's manual also contains information about the appropriate viscosity ratings of a motor oil.