10w30 vs 0w40 , thickness turning point ??

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10w30 is thicker than 0w40 in low temperature. 0w40 is thicker than 10w30 in high temperature. Am I right? Then... what temperature is the turning point ?? 0w30, 5w30, and 10w30 have same viscosity at high temperature. Am I right? Then... what is the minimum temperature that they have same viscosity ? I know they are different at winter start. Are they also different at summer start?
 
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 Originally Posted By: popolon
0w30, 5w30, and 10w30 have same viscosity at high temperature. Am I right? Then... what is the minimum temperature that they have same viscosity ? I know they are different at winter start. Are they also different at summer start?
What eljefino said. 0W, 5W,30 and 40 each represent a fairly wide range of viscosity, not an exact viscosity. Be aware that the data given by the mfrs is usually at 40C and 100C. Extrapolating that to temperatures well below 40C with a viscosity calculator will only give you a general idea what is happening.
 
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 Quote:
Then... what temperature is the turning point ??
What you're struggling with is viscosity index = VI. Fluids can change more or less over the same spans of temperatures. Water changes very little (in practical terms - nitpicker disclaimer ) over a broad temperature span. Molasses changes a great deal over a narrow temperature span. A fluid with some mythical VI of (something like) 950+ would have just about the same viscosity all the time over most functional temperatures that you encounter on the planet all the way up to what they encounter in the engine. Operating visc over all practical temps.
 
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