Base oil viscosity

Jan 3, 2020
Brittany, France
I was wondering, what are the typical viscosities of the base oils used in oils such as :

- Group III base low HTHS US 5W30
- Group III base high HTHS euro 5W30
- Group III base euro 5W40
- Group II/III base euro 10W40
- Group II or II/III HDEO 15W40
- Group I or II euro 15W40
- Group I or II 20W50

Is a euro group III 5W40 generaly using a similar base oil as the 5W30 version with just more VII? Is a group II/III semi synthetic 10W40 made from slightly thicker base oils than a group III 5W40? Thanks.

Edit : Also, what kind of VII loading to expect for each type of oil?
Last edited:
Feb 3, 2016
There is rarely just a single base oil used in finished engine oil. Typically it will be a blend of at least two base oils with the VII each with it's own viscosity profile. As for a "typical viscosity", that would depend on the conventions the companies follow in the formulation. Do they lean on VII to boost viscosity or try to minimize VII use? Do they try to maximize HTHS or just meet the minimums? Do they try to far exceed low temperature viscosity performance or just meet the minimums?

Beyond that, it depends on what VII is being used if a 5W-30 and 5W-40 would use the same base oil ratio with more or less VII to make the difference. Some VII's will effect the cold temperature performance more than others so boosting the 100C viscosity with VII alone may or may not knock the winter rating out of spec going from one grade to the next (and vice versa with high temp to low temp viscosity and VII effect)
Feb 27, 2009
down in the park
Look at the blending guides that sometimes pop up

here's one.


Notice how the 5w-30 (with in this case also a lower HTHS) uses significantly more spectrasyn 8, which is the heaviest baseoil in the mix, and less spectrasyn 6 which is the thinnest. Also uses less VII. Yet has a lower MRV and similar CCS. And lower noack aswell.

The same is happening with the 0W-xx oils.