Yet another viscosity question

[FrankN4]

I ask that you indulge me until I learn more.

There has been talk that Mobil 1 0W-40 is not a GREAT oil because it shears down to about a good 30 weight. What does this mean? Does it mean that the HTHS at 150C of Mobil 1 0W-40 is 3.7 and the HTHS of some top 5W/10W-30 is 3.2/3.5?

Also, is a Total Base Number of 8.0 bad for a synthetic oil or any oil for that matter?

Thank you all.

 

[Drivebelt]

Viscosity is measured in cSt -- with water = 1.0 cSt.

The various grades of oil have to be within a specified viscosity range measured at 100C (Celsius) = 212F.

30 … 9.3 -- < 12.5 cSt
40 … 12.5 -- < 16.3 cSt

Mobil 1 0W-40 is a 14.0 cSt motor oil (40 grade).

The base oils they use to make this motor oil actually start out around 5.0 - 5.5 cSt, much lower than 14.0 finished product because they have to meet the cold properties of the 0W specification in addition to the 40 grade (multi-grade). So they add these polymeric VII's (viscosity index improvers) to the (5.0 - 5.5 cSt) base oil to increase the viscosity to 14.0 to help the oil achieve these multi-grade properties.

The problem is, these VII's can shear down under high heat and stress (they're not near as stable as the oils themselves). When these VII's start shearing down (they break apart and lose effectiveness), the motor oil will loose viscosity -- getting perhaps as low as 11.0 cSt. This means if one measured the 0W-40 after use, it would no longer fall in the 40 weight range, but be in the 30 weight range.

The wider the viscosity spread, 0W-40 is wider than 5W-40 -- all things being equal, more VII's are needed to make the viscosity spread. A 5w30 would require less VII's than either of those. With less VII's, one is not depending as much on the VII's to keep the viscosity of the oil in the required range, so the oil is more shear stable/resistant -- i.e. not as prone to losing viscosity.

 

[Gary Allan]

Quote:
There has been talk that Mobil 1 0W-40 is not a GREAT oil because it shears down to about a good 30 weight.



Well, it may be a great or not so great oil, but I really never figured out the point of making a 0w-40 that sheared down to a 30 weight since they can just make a more shear stable 30 weight to begin with. The creation of the oil makes little sense from my view. Just blend it to just below the 40 weight divide and ..that's it. I just don't get it (visions of Tom Hanks in BIG "I don't get it".)

 

[FrankN4]

Thanks Drivebelt

That explains it. I thought that synthetic oils did not use/need viscosity index improvers. I thought a 0w, 5W, 10W-40 weight synthetic was actually a true 40 weight but would cold flow like a 0, 5, or 10 because of the propertities/characteristics of synthetic base stocks.

Thanks

 

[Drivebelt]

There are a couple of narrow viscosity spreads that can be done with synthetics an no VII's, like 10w30 (Amsoil has one, ACD 10w30) and 5W-20 if you use the certain synthetic base oils. But generally, the vast majority of synthetics these days have at least some VII's. The cold specifications these days are tougher that they were 20 years ago, so it's tougher to meet the viscosity spread these days with no VII's.