Whats the numbers mean?

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90
Location
Indiana
What does the first number mean like 10W-30 say to a 15W-50. And the last number? Why are synthetics different? Rando
 
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8,711
Location
Nothern USA
The numbers are easy. The higher the number, the thicker the oil. Where you have two numbers, one followed by a W it means the oil has additives in it that make it flow like that number when it is cold, and the other when it is hot. An oil thin enough to flow when it is cold, isn't thick enough to protect your engine when it is hot. The longer lives of today's engines may be largely due to the multi grade oils common now. By the time I had a car of my own to care for, I always used the multi grade oils, usually 10W-30, and now 5W-30. I will leave the synthetics to others.
 
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2,480
The numbers are an oil's grade and are simply an indication of an oil's thickness at temperature. First number refers to the oil's relative thickness at low temps (sub. zero)..eg. a 5 will flow better than a 10 or 15 at extreme cold (sub. zero). Second number refers to the oil's relative thickness at hot temps. The qualifying temp. of comparison is 100C. eg. a 40 will be thicker than a 30 or 20 at this temp..
 
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6,388
Location
Washington St.
quote:
Originally posted by Dr. T: The numbers are an oil's grade and are simply an indication of an oil's thickness at temperature. ....
Just so even the new folks know, the viscosity grade of an oil is not a quality grading. It could be more clearly described as a viscosity category or classification. A higher viscosity grade is not a better oil, it's just a thicker oil. What's the right thickness (viscosity grade) of oil?...the maker of the equipment decided on that for us. (Pls, take the 5W-20 debate elsewhere for now.) Ken
 
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418
Location
OR
What about straight weight oils like a 30 wt. Is there vscosity defined at low temp or high temp. (ie) If there were such thing as a 30W-50 would it behave like a straight 30 weight oil at 0 degrees F?
 
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450
Location
Louisville, KY
quote:
Originally posted by davefr: 1)What about straight weight oils like a 30 wt. Is there vscosity defined at low temp or high temp. 2)(ie) If there were such thing as a 30W-50 would it behave like a straight 30 weight oil at 0 degrees F?
1)Behaves like a 30 weight at 100C 2)Yes
 

MolaKule

Staff member
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21,584
Location
Iowegia - USA
Synthetic Oils are no different with regard to Multi-Viscosity classifications. Referring to SAE 10W30 weight oils: An oil is either "in-grade" or it is not. Some synthetics, such as Mobil 1 are on the 'thin' side of a 30 weight viscosity, while oils such as Amsoil and Redline come stock with oils on the 'thicker' side of a 30 weight viscosity. But all of the above referenced oils fall into the viscosity range for a 10W30 and are "in-grade." You want an oil to be thinner when cold so it will flow to the upper levels of the engine quickly, and thick enough to form a hydrodynamic film at higher temperatures where the oil generally thins out. Synthetic oils have naturally wide viscosity ranges and thin out less than do petroleum oils, and require less Viscosity Index Improvers (VII's) than do petroleum oils. High levels of VII's form thicker oils components when they shear into part of the sludge package. As Patman says, "they become useless little molecules floating around." I am still not sure if he was referring to oil rheology or someone on this list! [Big Grin] [ May 07, 2003, 12:08 PM: Message edited by: MolaKule ]
 
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