Regarding the so-called bright stocks, since they are nothing but thick (KV100 = 30+ cSt) Group I base stocks, I would consider them as part of the base oil
but not as any sort of a VM.
Apparently they are getting rare and expensive. This company has some data on them:Ergon bright stocks
I don't have much data on the PIB polymers but are they any different than other VIIs? High-molecular-weight PIBs increase the VI just like any other VII, don't they? They seem to be unpopular as VIIs today as better-performing VIIs are now available.
The reason I brought up the temperature dependence is because most people think that they increase the viscosity more at higher temperatures than at lower temperatures and that's the reason behind the VI improvement. In reality, the thickening effect is not necessarily strongly temperature dependent and actually thickening is more at lower temperatures than at higher temperatures for most VIIs. (See the figure above.) The reason they improve the VI is because the way VI is defined, it's based on actual base oils -- Group I to be exact -- and their viscosities change more rapidly with temperature as the base oils get thicker. So, if a VII multiplies the viscosity by the same number at all temperatures or even a slightly smaller number at higher temperatures, that's still an improvement in VI over how an actual base oil (such as Group I) would behave. PMA is an exception, as it thickens much more at higher temperatures, which could be used to make ultrahigh-VI oils (VI well over 200) but it requires 3 - 6 times the polymer content of other VIIs.
It's a very interesting subject and I'm glad it's a research interest of yours.
Nevertheless, I've also noticed that newer oils are trying to avoid VIIs as much as possible. For example, the recently introduced ACEA C5 0W-20 oils (Mobil 1 ESP x2 and Castrol LL IV) with Euro OEM specs seem to have very little VII. As ZeeOSix and I have been discussing, it looks like the best way to make an oil with high fuel economy, low wear, and low deposits is to start from a base oil as thick as possible with a VI as high as possible and use the least amount of VII possible
. Large temporary shear of the VII at 10^6 1/s is not necessarily a bad thing, which improves the fuel economy, as they all fully shear at high-enough shear rates anyway. Likewise, high thickening power is not necessarily a good thing, which makes the fuel economy worse, as the viscosity at lower shear rates gets higher. Again, apart from picking an oxidatively stable polymer (such as an OCP)
, the key to making a good motor oil seems to start from a base oil that is as thick as possible and has a VI as high as possible so that the VII content could be reduced as much as possible. In practice the availability of the base oils and economics determines what one can make.