A woman checks the oil level of her car with the dipstick.
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Oil lubricates the metal surfaces of your engine and stops them from grinding together and causing a lot of damage. The viscosity -- or thickness -- of the oil greatly affects your engine's performance. If the oil is too thick, it will flow too slowly between parts and your engine will get too hot. In the winter time, cold temperatures cause oil to thicken, but you can overcome this problem by filling your engine with an oil of a lower viscosity. Your owner's manual should tell you the ideal type of oil you should use, and it also might specifically suggest a thinner oil type depending on the season. Remember, most technicians recommend that you change your oil every 3,000 miles or once every three months.
