![]() ![]() In general, the wider the vis spread of a multi-vis oil, the higher the VI. VI is nothing more than an "indicator" of an oil's ability to resist viscosity change with change in temperature. Please feel free to correct me if I am mistaken. Otherwise the Noack would be higher like Mobil 1 and the viscocity at 100C would be lower like Mobil 1. I believe the VI Improvers in the 0w30 are probably the most shear stable in the industry. A lot of the difference in Noack is the amount of VI Improver that is burner off. Were the 5w30 to be made with the 196 VI basestock you would have seen a lower Noack than 6.9 %. That is why you see Amsoils base oil VI go up as the spread increases. That is if they use the same amount of VI improver. Higher VI base oils can cover a larger spread than lower VI base oils and therefore require less VI improver (modifier).Īlso, lower weight multis (5w30) require a higher VI base oil than higher weight (15w40) multis with the same spread (25). Your first sentence was correct but from there you went 180 degrees off. Ergo, the oil with the lower VI is the "better" oil. Hence, the oil with the lower VI uses LESS VI improvers. When it comes to synthetics, my theory is that if an oil of a certain grade, say 0w30, has a lower VI than the 0w30 of another brand, the oil with the lower VI is relying more on the VI of the base oil blend to achieve the vis spread than on VI improvers. Does higher viscosity index mean better? Or lower viscosity index mean better for cold starts? VI is nothing more than an "indicator" of an oil's ability to resist viscosity change with change in temperature. I notice most 10w30s are around VI=150 while 0w40s are around VI=190.
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