Choose your oil
Motor oils are defined by grade. Today's oils are generally multi-grade, which means that they are suitable for use in a range of temperatures.
For instance, when an oil is specified as "10W40," the first number is a measure of viscosity (resistance to flow) at lower temperatures, while the second number is a measure of viscosity at higher temperatures. A rating of "5W30" would indicate an oil that flows more easily at both low and high temperatures than a "10W40." In short, the higher the grade, the thicker the oil.
Many people prefer heavier oil (greater viscosity) for protection in hot weather and lighter oil (lower viscosity) for use in freezing temperatures. The best selling grades in the U.S. are 5W30 and 10W30 oils. If you're still unsure, read over the owner's manual again or consult a local mechanic.