The higher the VI is, the less viscosity drops between these two measurement points: The viscosity index is basically a measure of how much the oil's viscosity varies between 40☌ and 100☌. Namely how low in temperature it allows a test rig engine to turn over inside of a maximum electrical current threshold or something like that. The first number (0w, 5w, 10w, etc.) isn't really an indication of viscosity (not directly, anyhow) but of how well the oil performs in the CCS - Cold Cranking Simulator test. Sorry for the long post, I am tired and hope it made at least some sense. Check for HT/HS, and FP/PP and that will give you more clues to an oils quality. Look at the source/company, if it is known to have Grp.IV-V added so you can get an oil with a NATURALLY high viscosity index (almost all oils will have some VII's). A low natural VI oil Grp II with lots of VII's does not equal a great oil. To make a long story short, theoretically you could have an oil with a high VI and it be a lousy oil. There are varying qualities of these as well. you can be deceived because many companies add viscosity index improvers (VII's). Also what moleKule said about high VI oils today being better, he is right but (not to put words in his mouth) there are base stocks III,IV,V that have great VI's. Short answer, no, here is where TBN, quality of the base oil come into play so you are comparing apples to oranges there. On the second part of your question about lasting the same amount of miles. In reality, yes, both 0w30 and 10w30 should perform the same when up to temp. And they should both last about the same amount of miles assuming the Viscosity of each are comparable, right? So in reality the 0w-30 is going to do a little better on cold start up, but they should both perform the same after getting up to temp.
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