i agree 30w & 40w are different base stocks / just stating the first # is what it pours like
All of these numbers describe pour characteristics but at different temperatures/conditions. That is what viscosity means. Allow me a long winded example.
Mobil produces many different base stocks with different refining processes and different rheological (viscosity) characteristics. Lets take a look at one - SpectraSyn 10. Mobil describes this as a Group IV (PAO) grade 10 base oil.
Does that mean it is a SAE grade 10? No - in fact the SAE system has no grade 10. In this case grade 10 refers to its kinematic (pour) viscosity at 100C. In this case that is 10 cSt as measured in a simple gravity viscometer. In the SAE system that KV100 would be a grade 30 oil. If you look at the SpectraSyn 10 product data sheet below you will also see that particular base oil also has a VI of 137 and a CCS viscosity of 8840 cP @ -30C. That does not have a direct read across counterpart in the SAE system but it puts it very realistically in the neighborhood of SAE grade 10W and possibly even 5W.
So this virgin base oil containing ZERO polymers or other additives is already rheologically a SAE 10W, 30, and 10W30 grade oil all at one time. It is entirely possible to blend a full throated SAE 10W30 engine oil using a base oil of this type and nothing in the way of polymers. For a time Mobil, Amsoil, and a few others did exactly that.
If you go to the Mobil website and look at the full SpectraSyn catalog you will also see they have 11 different PAO base oils with KV100 ranging from a high of 100 cSt to a low of 1.8 cSt. Those viscosities far exceed the high and low end of the SAE grade ranges.
Why is that important? What do you thing happens if a blender mixes a smidge of the 100 cSt base oil, a bunch of the 10 cSt base oil, and another smidge of the 2 cSt base oil? If you choose carefully you can get a new rheological profile with better low
and high temeperature viscosity without resorting to any polymers,. Stretching the point a bit you might even be able to push the resulting KV100 over the lower limit for SAE grade 40 and the MRV/CCS viscosity below the upper limit for SAE grade 5W giving you a base oil with 5W40 rheology without using any polymers. This is why it is technically incorrect to talk about the viscosity of base oils in terms of SAE grades and futile to try and deduce the actual viscosity of the base oil(s) from the finished SAE grades.
The SAE J300 table and the grade label on the container tells you all you need to know about the viscosity profile of a finished oil. It also tells you next to nothing about the engine lubricating properties of the oil. That is very dependent on the other additive packages (wear , oxidation, detegents) that go into an engine oil. The API service classes are where you go to make those sorts of comparisons.
In practice mixing very expensive Group IV (PAO) and Group V (ester) base oils is not a cost effective way to blend a commercial multi-grade engine oi. So current practice is to use maybe three or more less expensive base oils from different base oil groups (primarily Group III) and use relatively small amounts of polymers to shift the rheological profile
both directions. You get the same SAE multi-grade rheology at a lower cost point.
Dan
