Originally Posted by HangFire
The oil with both the lower pour point, the higher viscosity at 100C and lower MRV viscosity at -40C has the larger Viscosity Index. That jives with my understanding of VI, the closer the oil stays to its running temp viscosity at the extremes, the higher the VI.
The kicker is that CCS Viscosity is a bit backwards between the two.
VI is simply calculated based on 40C and 100C visc. The higher, the less difference there is between those two temperatures; the less the viscosity changes with temperature. This is artificially manipulated using polymer VII's, which improve the VI of the base oil blends. This can be extrapolated both up and down to a degree, but visc calcs tend to lose accuracy much below 0C.
Now, you can use a heavier base oil blend with a reasonably high VI and require less VII to hit your target viscosity than a lighter base oil blend, which will in turn have a higher VI, because you used more VII.
This is consistent with your observations as to what's transpiring here, as the lighter base oil blend, padded with polymer to hit its 100C visc target, assuming similarly-based lubricants (IE, one isn't PAO, the other Group III), will have a lower pour point, lower MRV and higher VI.
Where a wrench gets thrown into these comparisons is when the two lubricants are quite differently based. M1 EP 0w-20 for example, being majority PAO, will have a lower pour point (-54C) than both of them, and likely better CCS and MRV numbers, despite having a VI of 173. In fact, if you look at M1 AFE 0w-20, which has less PAO in it than the EP product, but the same VI, it has an MRV of only 9,200cP, basically half of what it is for these GTL-based lubes.
