[QUOTE=Wisconsin S2k,Jul 27 2005, 03:17 PM]I saw this posted somewhere.

 

the letter "W" grade oils are based on a maximum low-temp viscosity. because the low temp in the witer, that is what the "W" designation stands for. everyone has a misconception on the letter "W". it actually stands for "winter" not weight. 5w-30 is used in colder climates such as below 0 degrees or even lower. 10w-30 is used in 20 degrees and above. as you can see they both have 30 (viscosity) so in which once they warm up they both will have the same weight at the end.

 

Viscosity has nothing to do with weight or specific heat. It is related to density, however, higher "weight" oils have about the same density as lower "weight" oils. It is confusing when people use the term "weight" when in fact they are talking about viscosity. Engine oils pretty much all weigh the same with specific gravities of about 0.9. Whether it's a straight SAE grade 5 or 30 or 50, they will all weigh about 1.9 lbs/quart.
Just a couple of notes.

Visc is related to the oils ability to form a film between the moving parts to keep them seperated. Too low a visc and the film will break down and the parts will touch resulting in abrasion, wear, gouging, microwelding, etc. Too high a visc and the oil can't get in and do its job of carrying away heat, debris and providing additive protection. A thinner(lower visc) oil, becuase more of it will flow past the moving parts, is able to take away more heat than a thicker(higher visc) which flows much more slowly past the point of heat transfer. So the reason an SAE 30 oil doesn't transfer as much heat when it's cold is because it isn't flowing as fast past the heat source.

With tighter tolerances on modern engines it's important that lubrication is provided upon startup. The multigrade oils act as thinner oils when cold which ensures flow into the bearings and other moving parts while the engine is cold. Once it heats up, it's important that it not follow that visc curve of a straight oil otherwise it will be too thin at operating temps.

Specific heat is the amount of heat per unit mass that a material or oil can hold. I dont think that this is a normal defined property of engine oils.

 

what he failed to mention and possibly understamd is that multigrade oils like 10w-30 satisfy atleast two different viscosity requirements each at a different temperature.


a 10w-30 oil satisfies the "10w" part at low/subzero temperatures ,and the "30" part is satisfied at higher temps. This means that the oil will flow like a lower viscosity oil at lower temps, a desired characteristic because colder temps make oil more viscous and it flows slower. And that it will flow like an oil with a viscosity of rating of 30 at high temps, again a desired characteristic because higher temps make the oil less viscous and lower viscosity means lower pressure, so to maintain proper pressure requirements a thickier oil which is made thinner by heat will actually be at a better viscosity at engine operating temps.


oils with rating such as 10w-30 satisfy more than one viscosity requirement , and each is satisfied at a different temperature.

single grade oils such as SAE 30 satisfy only one viscosity requirement. for the case of sae 30 it satisfies 9.3 to 12.5 cSt at 100'c
sae 40 --12.5 to 16.3 cSt at 100'c
sae 60 -- 21.9 to 26.1 cSt at 100'c

and not all the lower viscosity ratings are measured at the same temps.
10w is measured at -20'c , while
20w is measured at -10'c


im not a mech eng , so none of what i said should be taken as written in stone, and i hope someone will correct me if im wrong.

 

The numbers on an oil container refer to weight. Typically, the lower the weight, the lower the viscosity. The higher the weight, the higher the viscosity. The weight of a motor oil very much affect's the viscosity.



Is that so? Let me get this straight. So your saying that a base stock motor oil of 5, has the same viscosity of a base stock of, say, 30? Where did you get your info? I'm curious

This is not confusing as the number's are in fact, indeed referring to "weight" or "grade".

According to your two statement's here, all oil's weigh the same and they have the same viscosity.

This is true.



One last thing.

That is because a multi-grade oil is thinner. The base stock of a 5w-30 motor oil would be 5w. The base stock of a 10w-30 would be 10w.



When the oil temperature surpasses 100 degree's, the VII's (viscosity index improver's) increase the viscosity of the motor oil.

How can a multi-grade motor oil follow a visc curve of a straight oil

 

 

This is only confusing to people who don't have a 1/2 a clue when it comes to oil. I believe that even the most fundamental simpleton can figure out that when talking about oils, "weight" does NOT mean you take a jug of oil and put it on a scale to weigh it.
Before this thread happened, I would not have even conceived of the idea that anyone could be that uninformed. Also, before this thread happened, I would not have conceived that anyone would go through such a disertation to explain how "weight" of oil doesn't mean that it is "weighed".
Good GAWD! Was this truly necessary? Those who actually thought that oil is "weighed" really has to deal with many other issues about how the world works.
Come on, folks, we have very specific guidelines written in our owners manuals as to what type and rating of oils and fluids we can put into this car. Does it really matter for the vast majority of owners, if a thicker oil hold more pixie dust than a thinner oil? ........................ Perhaps if an owner were to go outside of the designated parameters as dictated in the manual, but then, those people tend to be extremely knowledgeable about what they're doing or they tend to be absolute morons. Do were really need to address either of those two groups?
Geez, please make the pain go away.

 

The numbers on an oil container refer to weight. Typically, the lower the weight, the lower the viscosity. The higher the weight, the higher the viscosity. The weight of a motor oil very much affect's the viscosity.




Is that so? Let me get this straight. So your saying that a base stock motor oil of 5, has the same viscosity of a base stock of, say, 30? Where did you get your info? I'm curious

No, I'm saying that they all weigh about the same. They all have widely varying viscosities.


This is not confusing as the number's are in fact, indeed referring to "weight" or "grade".

My remarks were in the context of the quote from the previous commenter who said that a 30 oil was six times "heavier" than a 5 oil. To me, heavier means it weighs more. His logic also wouldn't work with a 0 oil. Does a 30 oil weigh infinitely more than a 0 oil?




According to your two statement's here, all oil's weigh the same and they have the same viscosity.

No. My statement was that all engine oils weigh approximately the same. They have widely varying viscosities.


This is true.



One last thing.


That is because a multi-grade oil is thinner. The base stock of a 5w-30 motor oil would be 5w. The base stock of a 10w-30 would be 10w.




When the oil temperature surpasses 100 degree's, the VII's (viscosity index improver's) increase the viscosity of the motor oil.

A VI improver works across the entire range, it doesn't kick in at 100C. Typically a straight mineral oil has a VI around 100 - 120. VI improvers are added to boost the VI so that its visc doesn't change much over the operating range of the oil.


Here some links I quickly looked up regarding ExxonMobil engine lube oils. Note that the densities, are all about the same in the 0.85-0.90 range. Also note that the multigrades have much higher VIs than the single grades

http://www.mobil.com/USA-English/Lubes/PDS...0_1340_1350.asp

http://www.mobil.com/USA-English/Lubes/PDS...il_1_10W-30.asp

http://www.mobil.com/USA-English/Lubes/PDS...obil1_0W-40.asp

How can a multi-grade motor oil follow a visc curve of a straight oil

It can't. That wasn't what I was saying

 

But then, that's just "you", right? Most people who talk of oils know better. You are making assumptions that don't need to be made.

 

Again, it sound's as if you are saying that all multi-weight's, or if you will, multi-grade motor oil has the same viscosity.


Then you make this statement