Still don't get the 40c/100c Viscosity ratings

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When I think about a 5W-30 oil, it implies that it has the viscosity of a 5 weight at cold/ambient temps and the viscosity of a 30 weight at operating temps. So I look at a data sheet for a 5W-30 and it tells me that the [email protected] is 65, and the [email protected] is 11. So far, so good. I then look at the viscosity chart on this website and the numbers I see don't make sense for the 5W/40C end of things. The chart says that a 5W oil at 40C has a cst of around 18 to 20 - not the cst of 65 shown on the data sheet. If I take the cst of 65 @ 40C and move up the chart, it says it's equivalent to a 20W oil @40C. So is the 5W-30 in my example really a 5W-30, or is it actually a 20W-30 ? The numbers are pretty similar for all the 5W-30s. In fact, this descrepency exists for all multi-grade oils I looked at. The numbers all match quite well on the 100C end of the scale for the listed viscosities. It just the cold end that doesn't make sense to me. What gives ? Phil
 
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The first number is the viscosity at VERY cold temperatures. The viscosity above 0°C seems to be more dependent on the second number.
 

va3ux

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quote:
The problem with this common definition of a multi-viscosity motor oil, "like a 10 weight oil at cold temps and a 30 weight when hot", is that it implies that oil is thin at cold temperatures. That's just backwards, as oil thickens as temperatures decrease from the 100C(212F) point at which the SAE 40, SAE 30, etc. grades are defined
Blue99, I understand that. I know that 5W-30 is thicker when cold. But the whole darn naming format suggests that it has the viscosity of a 5W weight when cold, which is thinner than a 30 weight at the same temp. When I compare data for straight 30 weight, the 5W-30 certainly is thinner at 40C than the straight 30 is at 40C, but still a long way from the viscosity shown on the chart. I guess from your reply and the other ones here, maybe it is simply a matter of : 1) The 5W designation only pegs the low temperature Pump and Crank viscosities for that oil. 2)Above 0 deg C (or some other unknown, unspecified temp), the second number influences the physical characteristics more than the first number. Alrighty. Thanks .....Phil
 
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The 5W specification means it conforms to a viscosity range at -30C and a maximum allowable viscosity at -35C. As far as I know, a viscosity value at 40C is only for calculating viscosity index and not a property that determines the oil's viscosity rating.
 

va3ux

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rpn453 and ColdCranker, Thanks - both of your posts have filled in the blanks for me. The other thread had the information I was missing and I now understand. I followed the link to the Java viscosity calculator and I can now play around with some numbers for the oils I have on hand. Thanks again...Phil
 
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The problem with this common definition of a multi-viscosity motor oil, "like a 10 weight oil at cold temps and a 30 weight when hot", is that it implies that oil is thin at cold temperatures. That's just backwards, as oil thickens as temperatures decrease from the 100C(212F) point at which the SAE 40, SAE 30, etc. grades are defined. See the SAE J300 Viscosity Chart . Note that 5 weight or 10 weight do not exist in the current SAE viscosity classifications. "W" is most commonly described as an abreviation for "winter" and is the cold temp rating of an oil. So the bottle of 10W oil on the shelf in my garage, which I use as a general purpose oil can lubricant, is a minimum 4.1 cSt oil with a cold temp rating of 7000 cP max @ -25C. The 10W oil does not have a 100C maximum rating like the SAE 20, 30, 40 etc. grades do.
 
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rpn453 explained it correctly. The problem is that the Winter ratings are measured by a completely different method and at different temperatures, leading to confusion. Add to this HTHS viscosity, measured by a different method at 150C. Multigrade oils (excluding some synthetics) are blended from more than one base oil viscosity, plus polymers, plus pour point depressants. Much beyond the 40C/100C limits, I doubt if these fluids behave in a linear fashion.
 
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