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Consult your manual for the proper interval between changes. Change your oil more frequently than the manual recommends if you drive under adverse conditions. Most vehicles today are driven under some type of severe condition, such as city driving, hot or cold temperatures, towing or off-road driving. It's the single most important thing you can do for your car.
Never mix motor oils in your engine. If you need to add oil, use the same brand, formulation and viscosity. Only switch motor oils when having your oil changed. Never pour used oil down a drain or the ground. A single quart of oil can pollute up to 250,000 gallons of ground water. After changing your oil, store the used oil in plastic containers and take it to the nearest recycling center. Many automotive stores will recycle the oil without charge.
Let's start with the two basic types, conventional and synthetic. Under ideal driving conditions, conventional motor oil works fine, but most conditions aren't ideal. If you make frequent stops and starts, drive for extended periods or in extreme conditions, synthetic is a better choice.
Viscosity is the thickness of the oil. In a multi-grade, such as 10W-30, the first number is the viscosity at low temperatures. The lower the number, the more easily the engine starts in cold weather. To maximize lubrication at operating temperatures, look for a higher second number.