Oil Viscosity vs. Temperature

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This is elementary, I know. Oil thickens as it cools and thins-out as it gets warm/heated up, correct? Someone just told me "No", multi-grade oils are designed to behave the opposite because of the VIs in them. Comments?
 
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isn't that weird. 5-30 5 should really mean thinner and 30 should be heavier. instead its opposite. weird.
 

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 Originally Posted By: Bulli
isn't that weird. 5-30 5 should really mean thinner and 30 should be heavier. instead its opposite. weird.
That's because the "W" number is not a viscosity. It is a cold-weather pumpability rating. The oil is not a 5-weight when cold and a 30-weight when hot.
 
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 Originally Posted By: insomniac
This is elementary, I know. Oil thickens as it cools and thins-out as it gets warm/heated up, correct? Someone just told me "No", multi-grade oils are designed to behave the opposite because of the VIs in them. Comments?
Tell that "someone" to set a bottle of 5W-30 dino outside in 0 degree weather... and try and pour some of it. Then set it inside the house for a few hours... and try and pour it.
 
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Though the physical characteristics of the liquid is thicker when cold and thinner when hot, multi-grade motor oil is looking at the viscosity characteristics when cold and hot. When virgin 5w-30 is cold it supposed to act as a 5 wt and when hot act as a 30 wt base on industry tests. The weirder thing is that over time as motor oil is heated and cooled over and over again, it becomes thicker at room temp.
 
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A 5w30 is an oil that meets the SAE cold cranking and cold pumping specs of a 5W as well as the high temp and HTHS specs of a 30 weight. The ideal oil would have a VI of 1000 and a virtually flat vis to temp curve all the way from its freezing point to its flash point.
 
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 Originally Posted By: Bulli
isn't that weird. 5-30 5 should really mean thinner and 30 should be heavier. instead its opposite. weird.
Well, it does. What everyone ...and I MEAN EVERYONE ... leaves off when they're telling someone this is "It thickens like a 5 weight (if there was a 5 weight) and thins like a 30 weight. There's some holes in that way of expressing it ..but it's essentially correct. Everyone holds a bottle of 5w-30 at room temp and thinks "hmmm thinner when cold ..thicker when hot ..all based on that 75F temp perception. What they miss is that a 5 weight is very thick at the colder temps and a 30 weight is very thin at operating temp.
 
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This might help you out a little:
 Quote:
Temperature has a big effect on viscosity and film thickness. As a point of reference, one SAE grade increase in viscosity is necessary to overcome the influence of a 20°F increase in engine temperature. At a given reference point, there is approximately a 20°F. difference between viscosity grades SAE 30, 40 and 50. SAE 20 is somewhat closer to 30 than the other jumps, because SAE 30 must be 30°F higher than SAE 20 to be roughly the equivalent viscosity. In other words, an SAE 20 at 190°F is about the same kinematic viscosity as an SAE 30 at 220°F, which is about the same viscosity as an SAE 40 at 240°F. This approximation works well in the 190°F to 260°F temperature range. One might be surprised at the slight amount of difference between straight viscosity vs. multiviscosity oils with the same back number (for example, SAE 30, SAE 5W-30, and SAE 10W-30). If an SAE 50 oil at 260°F is as thin as an SAE 20 oil at 190°F, imagine how thin the oil film becomes when you are using an SAE 5W-20 and your engine overheats. When an engine overheats, the oil film becomes dangerously thin and can rupture.
Noria Publishing
 
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The article is from 2003. I used to think like that too. Oils is the SAE 20 range provide more than enough film strength in engines designed to use such oils. You'll have other problems if your car overheats, not the oil film strength at the journals.
 Quote:
Polymers are plastics dissolved in oil to provide multiviscosity characteristics.
No, a plastic is a polymer, but not all polymers are plastics.
 
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Multiweight is the opposite of strait weight oils in the sense it "fights" the normal tendencies to thicken and thin.
 
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 Originally Posted By: Auto-Union
Multiweight is the opposite of strait weight oils in the sense it "fights" the normal tendencies to thicken and thin.
That's not really accurate. A multi-grade oil thickens less than a straight weight as the temp goes down, but it still thickens. But it doesn't "fight" thinning as the temp goes up. A 5w30 and straight 30, if they are the same vis at 100C, will more or less thin at the same rate above 100C.
 
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 Originally Posted By: insomniac
This is elementary, I know. Oil thickens as it cools and thins-out as it gets warm/heated up, correct? Someone just told me "No", multi-grade oils are designed to behave the opposite because of the VIs in them. Comments?
The VII's keep a multi grade oil from thinning "as much" when it gets hot. They dont make oil get thicker when hot. 10w40 = SAE10 when cold and SAE40 when hot. SAE10 when cold is thicker than SAE40 when hot. In simple terms, as heat builds, the VII's expand thus slowing the thinning process to make the SAE10 act like a SAE40, but it still thins out, just not as much as a SAE10 would. When the engine is turned off, the oil eventually cools, and the VII's retract, thus keeping the oil from thickening "as much" as a SAE40 would otherwise, but it still thickens as it cools.
 
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 Originally Posted By: hominid7
10w40 = SAE10 when cold and SAE40 when hot.
Not quite correct. In fact, there is no such thing as SAE 10. - Scott
 
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There is such a thing as SAE10. I should have typed SAE10W. However for the purposes of answering the OP's question related to common passenger car motor oils, I simplified the explanation as much as possible.
 
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A straight 30 oils behaves like a 30 in all temperatures. A 5 -30 behaves like a 5 when cold, and a 30 when hot. The actual thickness of a 5-30 is thicker when cold than hot, but not nearly as thick as a straight 30 [when cold].
 
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To be technically correct, at 40 degrees C: SAE - GTX = PP The derivative of GC / 4 = OEM + K&N Vlls – Vls = UOA (PDS*AutoRX) + 5W30 = M1/OCI Sorry, momentary acronym overload...
 
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