I posted this at the end of a thread the other day, but have since added a few more oils to the data table and feel like it should be it's own thread. This is for dino oils only. "The higher the VI (viscosity index), the more multigraded the oil" (How to read a can of oil (Part 1), by John S. Evans B.Sc., WearCheck Africa). Here are some oils (Valvoline All Climate, Chevron Supreme, Exxon Superflow, Quaker State Peak Performance, Citgo Superguard, and Conoco Supreme) ordered by their VIs: 162 5w30 Valvoline 159 5w30 Quaker 159 5w30 Chevron 158 5w30 Conoco 158 5w30 Exxon 154 5w20 Citgo 154 5w30 Citgo 153 5w20 Exxon 151 5w20 Quaker 150 10w40 Conoco 148 5w20 Chevron 148 10w40 Quaker 148 10w40 Chevron 148 10w40 Valvoline 148 10w40 Citgo 147 10w40 Exxon 139 10w30 Quaker 137 10w30 Citgo 136 15w40 Valvolines 135 10w30 Chevron 135 10w30 Valvoline 135 10w30 Exxon 134 10w30 Conoco 125 20w50 Valvoline 125 20w50 Exxon 122 20w50 Chevron 121 20w50 Citgo 120 20w50 Quaker The blank lines are there to emphasize the bigger drops in VI as you go down the table. It is clear that the 5w30 is the most multigraded of these various grades (dosen't necessarily mean the most VIIs though), with the 5w20s following a close second. The 10w40w follow close behind the 5w20s. Then there is a big gap to the much less multigraded 10w30 and another big gap from that to the 20w50. I threw in the 15w40 although it is more like a heavy duty engine oil because it shows where 15w40 might fit, although I suspect with more data we would find the 15w40s would more likely be between 10w30 and 20w50. If the viscosity index improver additive levels are roughly comparable to the VI (I know part of that is the natural VI of the base oil, but all else equal...), then 10w30 is the way to go except when extreme cold dictates thinner oil. The idea of running 5w30 or 5w20 during hot summer weather appears absurd from this viewpoint. And the much-maligned 10w40 ("too much viscosity spread," "suffers permanent viscosity collapse," etc.) appears actually to be more stable than the ever-popular 5w oils. Even when you look at each of the oil brands above separately (removes some, but not all, the unknowns) each separate brand still gives the same relationship I have described above. However, even within a single product line there could be big variations in the mix between 5w20 and 20w40. All else equal, the 10w40 doesn't look so bad as it often is made out to be. And I believe today they have some very robust VIIs available, though I would not so much expect them in the bottom line oils. Likely the Chevron Supreme has more natural VI than some of the others I've listed, in which case their 5w30 may have less VII than another brand's 10w40.