Originally Posted By: Tempest
To the best of my understanding, VII is more volatile than is the base oil. So the more VII, the higher the NOACK.
No, this is wrong. The reason you might see higher Noack (note: not all CAPITALS) with higher VII dosage is because you would, for a given KV100, be starting with a lighter base oil, which will be more volatile (on a like-for-like basis). So, light base oil + lots of VII = high Noack. VII has a very large molecular weight compared to base oil and will be more likely to thermally decompose than evaporate.
Originally Posted By: Tempest
Also, more VII will show a higher 100C temp (relative to lower amounts of VII), so NOACK can be a rough way to scope out VII concentration in an oil.
As stated above, you can't relate VII dosage to Noack. Noack is almost exclusively determined by the base oil mix, both its type and its viscosity (which is, itself, related to it molecular weight). One benefit of synthetics vs minerals is that a mineral is a broad mixture of molecules, whereas a synthetic is a much purer substance. The mineral has a mix of molecules, some 'big', some 'little'. The little ones will tend to evaporate more readily.