Viscosity is confusing me

Joined
Mar 25, 2008
Messages
733
Location
Kansas City Metro
I have the following viscosity chart and I can't remember where I got it. I was looking at some data sheets on some Mobil 1 oils and either that chart is wrong or I am reading it wrong. For Mobil 1 0w-40 cSt @ 40* C is 78.3 cSt @ 100* C is 14 According to my reading of the chart, this would be a 20-30 grade oil. For Mobil 1 Racing 4T cSt @ 40* C is 86 cSt @ 100* C is 13.8 According to my reading of the chart, this would be a 20-40 grade oil. So what am I doing wrong. I am probably showing my noob status here so go easy on me.
 
Joined
Dec 21, 2003
Messages
12,385
Location
Northern CA
The 40C viscosity is for reference only. just use the 100C viscosity. The change from 40C to 100C is used to calculate viscosity index and general interest but is not directly used for engine oil SAE viscosity rating.
 

JSP

Joined
Jul 31, 2008
Messages
127
Location
WA, USA
The chart is only for the second number, ie. viscosity at opertating temperature approx 100C. So both of the oils you listed are Xw-40 according to the chart.
 
Joined
Apr 5, 2004
Messages
9,427
Location
Pensacola & Vero Beach FL
 Originally Posted By: tom slick
Gotta love that straight forward SAE system! There isn't any part of the way they rate oils that isn't confusing.
Yup, one of the very few occasions when a double negative is really necessary to describe the way things are. . . On a more serious note, remember that the "w" part of the rating is not really a "viscosity," at least not in a direct sense. It is a result of the oil being able to pass certain tests. So, for example, an oil that's a tad thick, but stays liquid to very low temps, may be rated at a lower "w" rating than a thinner, cheaper oil that starts to solidify (and thus becomes unpumpable) at higher temps.
 
Joined
Mar 2, 2009
Messages
2,371
Location
WA
 Originally Posted By: dsmith41
I was looking at some data sheets on some Mobil 1 oils and either that chart is wrong or I am reading it wrong.
See the last note at the bottom of the chart. As others have said, the SAE oils are rated at 100 deg C ... only the ISO and AGMA are rated at 40 deg C.
 
Joined
Dec 21, 2003
Messages
12,385
Location
Northern CA
 Originally Posted By: ekpolk
 Originally Posted By: tom slick
Gotta love that straight forward SAE system! There isn't any part of the way they rate oils that isn't confusing.
Yup, one of the very few occasions when a double negative is really necessary to describe the way things are. . . On a more serious note, remember that the "w" part of the rating is not really a "viscosity," at least not in a direct sense. It is a result of the oil being able to pass certain tests. So, for example, an oil that's a tad thick, but stays liquid to very low temps, may be rated at a lower "w" rating than a thinner, cheaper oil that starts to solidify (and thus becomes unpumpable) at higher temps.
Not being a smart [censored] here but the right hand number isn't a viscosity either, it's the result of an oil being able to pass a certain test at 100C. The left hand and right hand numbers are the results of different viscosity tests. Viscosity is the resistance of a liquid to shear forces and therefore to flow. The W pumpability and cold cranking tests measure viscosity, just in different ways than the 100C tests. The results of both are expressed in units of viscosity. If I were made Emperor of the world, engine oil viscosity would be given in a form like "-35CS 12" where the -35 was the Cold Start rating in degrees C when evaluated by some standard cold start test to some standard actual viscosity and the 10 was the actual viscoity in cSt at 100C.
 
Joined
Sep 5, 2006
Messages
19,479
Location
Chicago Area
dsmith41 - The chart doesn't show the viscosity/temp ratings at 0 deg F. This is the first number of a multigrade oil system. Your 20 weight conclusion started in the middle somewhere.
 
Joined
Apr 5, 2004
Messages
9,427
Location
Pensacola & Vero Beach FL
 Originally Posted By: XS650
. . . Not being a smart [censored] here but the right hand number isn't a viscosity either, it's the result of an oil being able to pass a certain test at 100C. The left hand and right hand numbers are the results of different viscosity tests. Viscosity is the resistance of a liquid to shear forces and therefore to flow. The W pumpability and cold cranking tests measure viscosity, just in different ways than the 100C tests. The results of both are expressed in units of viscosity. If I were made Emperor of the world, engine oil viscosity would be given in a form like "-35CS 12" where the -35 was the Cold Start rating in degrees C when evaluated by some standard cold start test to some standard actual viscosity and the 10 was the actual viscoity in cSt at 100C.
At least the hot number makes an attempt to be an actual reflection of viscosity -- the real thickness of the oil. That said, I still think the system is awful. We don't buy gasoline in a range of octanes -- you get 87, 89, 93, etc. (or whatever is offered in your area). If you buy milk, you get 2%, fat-free, etc. But with oil, we just get a range. They should just sell oil based upon it being so many centistokes, hot and cold. But I won't hold my breath. . .
 
Joined
Mar 2, 2009
Messages
2,371
Location
WA
 Originally Posted By: XS650
Not being a smart [censored] here but the right hand number isn't a viscosity either, it's the result of an oil being able to pass a certain test at 100C.
Sure it's the viscosity ... defined @ exactly 100 deg C. That's why there is a viscosity unit of measure (cSt = centistoke) associated with the SAE weights ranges on the chart.
 
Joined
Mar 2, 2009
Messages
2,371
Location
WA
 Originally Posted By: ekpolk
At least the hot number makes an attempt to be an actual reflection of viscosity -- the real thickness of the oil. That said, I still think the system is awful. We don't buy gasoline in a range of octanes -- you get 87, 89, 93, etc. (or whatever is offered in your area). If you buy milk, you get 2%, fat-free, etc. But with oil, we just get a range. They should just sell oil based upon it being so many centistokes, hot and cold. But I won't hold my breath. . .
Gasoline octane probably has some kind of rating range too ... anything in the “defined 87 range” is called “87 octane”. Since there are so many formulations of oil, you obviously can't make a "30 weight" hit an exact cSt viscosity at 100 deg C; so they came up with the SAE weight ranges. IMO, it would be confusing (except for the oil maniacs in the world) if they had an absolute cSt rating for all oils. This info can be had on most oil manufacture's websites if the maniacs really want the tech info.
 
Joined
Mar 2, 2009
Messages
2,371
Location
WA
 Originally Posted By: widman
That is the old chart. This one went into effect in 2005
Looks like the viscosity (cSt range) was shifted downward (ie, thinner oil = same weight rating of the older chart). That chart needs finer graduation markings on the cSt scale for easier readability.
 
Joined
Dec 21, 2003
Messages
12,385
Location
Northern CA
 Originally Posted By: SuperBusa
 Originally Posted By: XS650
Not being a smart [censored] here but the right hand number isn't a viscosity either, it's the result of an oil being able to pass a certain test at 100C.
Sure it's the viscosity ... defined @ exactly 100 deg C. That's why there is a viscosity unit of measure (cSt = centistoke) associated with the SAE weights ranges on the chart.
It's not a viscosity, it's an SAE grade that represents a range of viscosities, just as the W rating is an SAE rating that represents a range of viscosities measured under different conditions with different equipment using different units of viscosity. EKPolk. The ISO rating system for viscosity of industrial lubricants would be a good model for a high temp rating. One of the charts further up this thread shows that. Thet use actual viscosity in cSt with a plus/minus tolerance band, but unlike SAE ratings, the adjacent ratings don;t share a common boundary.
 
Joined
Mar 2, 2009
Messages
2,371
Location
WA
 Originally Posted By: XS650
 Originally Posted By: SuperBusa
 Originally Posted By: XS650
Not being a smart [censored] here but the right hand number isn't a viscosity either, it's the result of an oil being able to pass a certain test at 100C.
Sure it's the viscosity ... defined @ exactly 100 deg C. That's why there is a viscosity unit of measure (cSt = centistoke) associated with the SAE weights ranges on the chart.
It's not a viscosity, it's an SAE grade that represents a range of viscosities ...
Yes, it's semantics ... what I meant was it's directly related to the oil's viscosity. The "SAE grade" is directly tied to specific range of kinematic viscosity. They use the term "SAE Grade" to define a range of the oil's viscosity at 100 deg C. The oil's formulation will determine it's viscosity at 100 C, and hence it's "Grade". I think we're on the same page.
 
Joined
Apr 5, 2004
Messages
9,427
Location
Pensacola & Vero Beach FL
Bottom line for me is that I'd really prefer to buy oil the same way I buy gas. I'd like to be able to choose an oil that's directly labeled as being 8.5 cSt at 100C, rather than having to first ID the 20wts, and then check each for it's actual vis when hot. What's so hard about that? Would you like to find fuel pumps labeled, "regular," "mid," and "premium," and then have to go to a book or the internet to find out what the actual octane of the "regular" at your local gas station might be? Tantalizing note, FWIW: of all the oils I've looked at through their PDSs, Schaeffer is the only one (may be others that I've not noticed) that lists a range of viscosities on its PDSs. I really don't know what this means. Is Schaeffer's process less accurate than that of others? Or is oil brewing a process that inherently produces batches of product with "natural" variation, and only Schaeffer is being fully honest about it???
 
Joined
Mar 2, 2009
Messages
2,371
Location
WA
 Originally Posted By: ekpolk
Bottom line for me is that I'd really prefer to buy oil the same way I buy gas. I'd like to be able to choose an oil that's directly labeled as being 8.5 cSt at 100C, rather than having to first ID the 20wts, and then check each for it's actual vis when hot.
Yes, I agree ... the cSt info @ 100 C would be good additional info on the bottle's label for the oil manics in the know. I doubt the oil industry would go to that standard without some major pushing for it.
 
Top