Originally Posted by Mad_Hatter
VI refers to resistance to thinning over a broad(er) range of temps and not necessarily thicker.
The real trick of the VII is that it thickens the oil without substantially altering the cold-cranking (CCS) viscosity. Yes, it will alter it, but since VIIs temporarily shear during the cold-cranking test, it won't be dramatic. The cold-pumping viscosity MRV will change more with the VII content though, as it's a low-shear test.
So, what I'm trying to say is that having a higher-VI oil won't help you when the temperatures get extremely high or extremely low. Performance at extreme temperatures are determined by the HTHS and CCS/MRV, not the VI. In fact, as I explained earlier, higher-VI oils will increase wear because the oil will have a lower HTHS100 (as opposed to HTHS150), because of the high VI ("resistance to thickening" as you called it -- 100 C is closer to typical oil temperatures than 150 C), and it also will exhibit more temporary shear in extreme conditions such as wide-open throttle because of the higher VII content.
Higher-VI oils will have higher fuel economy though, as HTHS100 will be lower, and HTHS100 and HTHS150 will both be lower in more operating conditions when shear rates increase beyond the 1,000,000 per second where they are reported, as higher VII content will exhibit more temporary shear.
In summary: For wear and deposit control, you want a low VI and low VII content, and for fuel economy, you want a high VI and high VII content.