Bror, I suspect the truth is somewhere between your two rankings.
What we really need is the actual 100C viscosity of the base oils used in each oil and then take the ratio of that with the 100C viscosity of the polymerized multigrade oil. In which case, maybe the 5w would not be much worse than a 10w as the API chart shows fairly narrow viscosity ranges for those two grades.
Another thing: Since I don't know how VIIs act on the base oil, is whether to do a straight ratio (as in your example) or look at how many times the viscosity must be doubled. The doubling, using the oversimplified SAE grade numbers, would look like this:
20w50 = 1.25 doublings
15w40 = 1.33 "
10w30 = 1.5 "
10w40 = 2.0 "
5w20 = 2.0 "
5w30 = 2.5 "
The ordering remains the same as with the ratio method (which makes sense), but since 5w30 is getting a bad rap by these crude methods, lets look as actual 100C viscosities. Assuming the base oil viscosity actually is withing the "w" number's API range, we'll give our 5w30 a base oil 100C of 4.0 and our 10w40 a base oil 100C of 5.0, with polymerized 100C viscosities of 10 and 14, respectively. Then the ratios are:
5w30 = ratio of 250%
10w40 = ratio of 280%
So now the 5w30 is looking better than a 10w40. But what if the 10w40 base oil is closer to the 5w range, say 4.5, then we get a ratio of 311% and the 10w40 is looking worse.
[ December 27, 2003, 01:46 AM: Message edited by: TallPaul ]