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Oil viscosity is a well misunderstood concept. A higher viscosity or weight results in a thicker, more difficult liquid to pump.
The interesting point is that when your engine is warmed up completely, 30 weight, 5W-30, 10W-30, 15W-30 all have the same viscosity. The difference is the viscosity when the oil is cold.
Standard 30 weight becomes very thick when cold. So thick that your engine parts will not be lubricated well. Thiner oils such as 10 weight are okay when cold, but run too quickly to get good lubrication at full operational temperature.
The answer is viscosity modifiers. They take thin oil and add special molecules which shrivel up when cold but stretch out when warm. These modifiers make the viscosity be like 10 weight oil when below zero yet 30 weight oil at full operating temperature. They call that oil 10W -30 where the W is for winter.
5W-30 oil is simply 5 weight when cold, but 30 weight when warm. Similarly, 15W-30 is like15 weight when cold.
Always use the exact same second number as your manufacturer recommends such as 20, 30 or 40. Try to use the lowest first, or W number as possible. This will give you the best lubrication no matter what the temperature even if you live in a warmer climate.
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