Base oil quality is not dictated by viscometrics and volatility alone. Other quality factors include oxidative stability, thermal stability, coking tendencies, additive solubility/flexibility, dispersancy, lubricity, seal compatibility, and hydrolytic stability.
Viscosity Index and volatility are decent quality indicators for branched hydrocarbon base oils in that high VI and low Noack also imply oxidative stability, but not all base oils are branched hydrocarbons. For example, Alkylated Naphthalenes (ANs) tend to have low VI and medium volatility, but are very oxidatively stable with good additive solubility. Likewise, some renewable esters have very high VIs and very low Noacks, but terrible oxidative stability. PAOs have excellent oxidative stability but are cokers in thin-film, high temperature environments and have very low additive solubility. Esters are very oxidatively stable with low Noacks and excellent additive solubility and cleanliness, but some can swell seals excessively and have lower hydrolytic stability. Most base oils are blends, often containing different chemistries designed to balance desired properties, so trying to characterize “overall base oil quality” in a simple formula seems unproductive to me.
Not only are base oil blends complex, but the additives further cloud the usefulness of simple quantitative quality formulas. Additives are arguably more important than base oils with respect to finished oil quality (which is all that really matters) in that they dominate wear control, cleanliness, oxidative stability, and rust & corrosion, and influence fuel economy, viscometrics, and volatility. The effects of additives greatly blur the usefulness of attempting to rank base oil quality from finished oil data.
From my point of view, viscometrics are a pass/fail criteria, i.e. HTHS and low temperature flow are either acceptable or not for my engine, driving conditions, and environment. For my use, a 0W or 5W-20 or 30 are fine, and I have no need to split hairs with further delineation. And Noacks passing GF-5 are also fine.
We all like to have rules of thumb for making decisions, especially those that translate into numerical rankings that are easy to follow, but often real world usefulness is lost in the quest for simplicity.
More important, such ranking lists can be counterproductive in that they lure people into making decisions based on a ranking that does not correlate to what really matters, which is matching a finished oil quality to your needs.
Tom NJ/VA