Ok will prob ask the mother of all stupid Q's

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Low temp vis. SAE30 can be thought of as a 30W30. It acts like a 30 wt oil all around. 0W30 acts like a zero wt when cold (thinner than SAE30) but acts like a 30 wt when hot, same as SAE30.
 
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Your confusion is understandable and is compounded by those who post here who insist on referring to oils such as 0w-30 as a "30 weight oil".
 
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Just think of it this way: A straight SAE30 will not (or may not) meet the cold flow requirements for a 0W, or a 5W or a 10W. But a 0W-30 could be labeled as a SAE30 if an oil company wanted to do so. (Misleading, no?) But both should be within the same viscosity range at 100°C.
 
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SAE30 is a monograde oil versus 0w-30 being a multigrade. Any label that has the 'w' means it's a multi-grade, also known as multiviscosity. From the glossary off bobistheoilguy home page: 'Monograde' (single grade) is a term used to describe an oil when its viscosity falls within the limits specified for a single SAE number.(SAE Standard J300) 'Multigrade' is a term used to describe an oil for which the viscosity/ temperature characteristics are such that its low temperature and high temperature viscosities fall within the limits of two different SAE numbers. (SAE Standard J300) the threads above reworded these definitions in plainer english. Monogrades are commonly labeled just 30, SAE30, 30HD, SAE30HD, and so on with no 'w' in the label.
 
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In addition to all that was written above, straight-weight (aka "monograde") oils usually do not (or maybe never do) contain Viscosity Index Improvers (VII's) or Friction Modifiers. On one hand it means they don't flow as well as their OW-, 5W-, 10W-, 15W- or 20W- brethren at low temps, but on the other they do not shear down to a lower viscosity with use, as do the xW- versions, and they are usually less volatile, if they are made with Group II basestocks, as are Delo 400 straight grade oils.
 
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