The 0w-xx designation is not a base oil viscosity, it's just a temperature performance target, as shown in the chart.
Blending is not as simple as just choosing a base oil that meets the Winter rating (like a 2cSt Group III) and just pumping it full of VII, as there are numerous parameters that must be met in order to satisfy performance targets.
Mobil makes numerous PAO bases that can meet the 0w-xx performance target, in varying viscosities. When blending using them, and other bases, the ones you might use will depend on what the application is. An 8cSt PAO can be blended straight to make a 0w-20 (no VII) for example, but what we see from the VI on the finished products is that this approach isn't taken. AMSOIL famously sold a 10w-30 with no VII for a while, not sure if they still do.
An oil like a Euro 0w-40 has to meet strict Noack limits, which means heavier bases are going to be used vs say a GF-5 0w-30 that can have a Noack of 13% when economics are brought into the picture. You'll see the opposite of that in one of the tables I'll show below where a "no holds barred" approach is taken in blending a PAO + ester 0w-30 which of course has less VII than the similarly blended 0w-40.
We can see some of the Japanese fuel economy OEM lubes used wickedly light bases, which you can tell by the Noack volatility:
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Here are the Shell GTL bases, you can see that they have excellent CCS values:
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One could blend the 5cSt base with the 8cSt base to get an acceptable 0w-20 for example that would need very little VII.
Here's a Mobil chart showing that you can use a blend of Group II+ and Group III to drive down the cost of your 0w-20:
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Another few examples from the Mobil blending guide:
Group III with PAO, you can see they use more, lighter PAO in the 0w-40 than the 0w-30 and more VII:
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PAO with Group II+ this time, similar approach, but more PAO:
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Straight PAO with 10% ester, clearly a heavier base oil blend than the other two shown above:
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And another chart that shows that the base oil selection changes depending on the grade (this is just PAO + ester). Now consider that the Noack on M1 0w-40 is actually 8.8% (lower than shown below) and the majority of 0w-20 and 0w-30 lubes on the market are not blended like shown in this table. Because a 0w-20 blended with PAO needs VERY little (or none if you are being adventurous) VII, you can use a base oil blend that gets you extremely close to your hot viscosity target. On the other hand, the 0w-30 and 0w-40 will need more VII for the same type of base oil blend and subsequently need to use a lighter blend as a starting point to retain their cold temperature performance with the VII added.
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As I noted at the beginning, these are "ideal" blends, using expensive bases. In reality, none of the 0w-20 or 0w-30's you buy are going to be blended like the above except maybe M1 EP 0w-20, which is ~70% PAO. They'll be more like the Mobil EHC example with the lighter Group III (and potentially Group II+ added) bases, with PPD's, and a heavier VII treat, and this is typically reflected in the Noack.
That's something Shell has done well with, as their GTL bases have very low Noack numbers, even the light bases. The Japanese oils, like TGMO, with very high VI's, use very light bases and a lot of VII to achieve that high VI. The natural VI of even PAO isn't very high, so how you achieve a "stratospheric" viscosity index is by using very light bases and a high VII treat rate. The 0w-20 in the table above for example has a VI of only 162.