How Important Is Viscosity Index ?

[Idrinkmotoroil]

In looking at two Mobil1 oils in the 30 weight class which would be acceptable for my engine, one has a higher HTHS (3.17 vs. 2.99)number but the other has a higher viscosity Index number (167 vs, 147). Just wondering what is more valuable, a higher HTHS number or viscosity index. Also how important are Viscosity Index numbers ?. Any opinions welcomed.

 

[brian fix]

well depending if synth. or mineral,,, synth. has a natural VI or less chance of moving around as heating cooling stability in its base ,mineral base oil depends on alot of refining technics and sourse from the ground ,in other words the synth. can be built to a higher VI per base componets,and mineral needs added componets as a additive to base,nothing over say 200 Vi may be a point of debate,,,many blenders and base manuf. do a really great job on this ,,,multi-vis are generally higher and single wt are lower ,,stay away form forumalated oils with bases lower than 98...........BL

 

[Blue99]

Viscosity Index is really only the rate of change or slope of the line of the viscosity values as graphed from the 100C value and the 40 C value.

Comparing the VI index of two 5W-30's that are 10.5 cSt @ 100C, if one is 158 VI and the other is 152, the higher VI index will stay a little thinner as the temperature drops.

It's just an indication of viscosity performance and not really a reason to rate one oil over the next.

 

[TallPaul]

I think it is pretty hard to judge an oil on viscosity index when you have a range from high end synthetics that have no viscosity index improvers and so show the oil's true viscosity index, to a cheap dino of a wider spread that may have a whole lot of viscosity index improver additive padding out that viscosity index number.

Instead, compare oils on 100C viscosity, HTHS viscosity (if available) and cold properties (preferably cold cranking viscosity and cold pumping viscosity, not pour point). This will give you much better criteria to select an oil than viscosity index.

 

[JohnBrowning]

Go with the better HTHS if they are both truly synthetic.

 

[Idrinkmotoroil]

thanks for the great replies, looks like I was putting too much into viscosity index numbers by the sounds of things. It really helps to know exactly how these specs are calculated.

thanks !