Read your user's manual.

Use the paper or CD copy you received with the car.

Also you can go online to the manufacturer's Internet site and search for their user's manual for your make and model.

Select the user's guide that has an engine type similar to the engine in your car to learn what grade is recommended for various conditions.

Also you can check with your local dealer's service department.

Make sure to ask three possibly knowledgeable service writers and/or mechanics and hope that at least two provide the same answer.

Sam, TX