Which Has More Viscosity Index Improvers?

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Oct 22, 2003
By Detroit
I am thinking about how the typical DINO OIL range of grades (5w20, 5w30, 10w30, 10w40, 15w40, and 20w50) vary quite a bit in the amount of viscosity index improver polymers added. "They" always tell you to avoid a grade spread greater than 25 (e.g., avoid 10w40), but a heavier base oil will spread with VIIs better than a light base oil. Therefore, it seems to me that Dino oils in general would line up in the following manner: LEAST VII's: 20w50 NEXT STEP: 10w30 and 15w40 NEXT STEP: 5w20 and 10w40 MOST VII's: 5w30 I basically looked at multiples of the front (w) number to the back number, but also looked at a straight Dino oil within brands and the given Viscosity Index number for each of those oils. Anybody have input that might support/refine this list?
Couple the higher VI's, with a low spread in a 5w20, and I would imagine that it has the lowest amounts of VII's. I would imagine it would have to be that way, to make a 20 weight shear stable, as I would not want one to shear to a 10. [Frown]
I think I've asked this before but I'm not sure I've gotten an entirely satisfactory answer. What is more significant, the total amount of points spread or the percentage of the spread? For example, if stability rankings were by total points, this would be the order from least stable to most: 10W40 30 20W50 30 5W30 25 15W40 25 10W30 20 5W20 15 But if the spread was measured as a percentage (ratio of hot weight to cold weight) they would be ranked in this order: 5W30 600% 10W40 400% 5W20 400% 10W30 300% 15W40 267% 20W50 250% A major difference in rankings! [Eek!] [freaknout] --- Bror Jace
Originally posted by Brett Miller: I would bet the 5w20's have less VI improver than most oils, especially 20w50 [Patriot]
Yup. The 5w20 oils out there no doubt have the least amount of VI improver of any conventional oil. The inherent high VI of the Group III used coupled with the narrow vis spread are the reason.
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 ]
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