There are not too many reasons to switch from the recommended oil in the owner's manual, but that oftentimes doesn't make buying oil any easier. Understanding the different numbers and ratings on motor oil packaging will make you a better consumer and can help keep your car running smoothly. Viscosity represents a fluids thickness, or resistance to flowing. A high viscosity is less likely to flow smoothly because it is thicker (yogurt is more viscous than milk, for example). There are two numbers for viscosity of an oil, represented in combination, like 10W-30 or 20W-50. The first number, with the W, is for the winter temperature of the oil. This tells you how well it flows in cold weather when it the oil is thicker. The second number is how well the oil maintains its thickness in heat.  The first number should be at least 5W or lower if you live in very cold areas (see your owner's manual), as oil can get too thick for the car to start if the winter viscosity is too high. Your owner's manual will have a recommended viscosity rating for your car. If there is only one number, as found in some older cars, your car uses a "single-weight" oil. This will help make sure your car stays healthy and under warranty. Each motor oil has several different certifications possible, from the starburst-shaped API to ILSAC recommended guidelines. Make sure whatever oil you use meets the standards put forth by your manufacturer. Some certifications change as the get more modern. The current API designation is SL, though it has been SJ and SI before. Again, refer to your manual for your car. Though more effective, synthetic oils are also more expensive than their natural counterparts. Blending oils, however, is perfectly acceptable, so you do not have to shell out for pure synthetic oil if it seems too expensive. If your car has been running fine on a single weight oil (one viscosity rating), you don't want to switch now. This could cause sludge and grime that built up in the engine to dislodge and cause problems. Stick with...

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