There have sure been a lot of threads on this topic since 0W40 appeared. Many posts argue that unless you are a seriously hard core track person (who will use 15W50 or even heavier), continued use of 5W30 is dumb even for drivers who are 100% street and whose oil thus essentially never gets above the lower middle 200’s. A few of those “5W30 is dumb” posts will acknowledge that if you run all the way to the oil life monitor limit before changing oil, using 0W40 introduces risk of VI improver breakdown. But they dismiss that by saying practically no Vette drivers run that long between oil changes. Color me dumb, because once out of warranty, being a 100% street driver with no short trip bad habits, I run 5W30 full synthetic using old fashioned personal limits of 12,000 miles or 18 months, ignoring the calendar time limits of current monitors. And I do so having spent a career in the oil industry with access to a ton of actual oil testing in real automobile engines run in long duration testing (which I unfortunately cannot share with you beyond my summarized posts is this and other forums).

A quick review of the VI improver problem is to remember the octopus analogy. VI improver molecules work by having long tentacles that wrap around their central body at lower temps, but unfurl and stick straight out as temp increases. When the arms stick straight out, they interfere with flow of base oil molecules, just like you would if you stuck your arms out while walking through a crowd of people. That flow interference counteracts the tendency of the base oil itself to thin out as temp increases. Trouble is, the VI improver arms can shear off, and when that happens, the effective upper number in the oil’s weight can degrade to about 25 higher than the W number, or 0W25 for the 0W40 oil.

There’s one more potential problem with VI improvers that I’ve never expressed on this forum before now. I’ll admit up front that the effect is almost certainly very small even if it exists, and I have no data to prove it exists at all. But since so many people cite such flimsy evidence to argue for wide spread oil with VI improvers, I figure I should be able to offer some flimsy thoughts to be against it. Remember the prior paragraph that VI improvers work by flow interference of a few molecules, not by magically improving the base oil molecules themselves. A wide spread oil might have 5% VI improver molecules. So 19 out of 20 molecules are the same old 25 spread molecules as they always were, while 1 out of 20 molecules is an octopus with arms sticking out and interfering with free flow and passage of the other 19 molecules. My intuition thinks that can’t be as effective as a lubricant as it would be if all 20 of the base oil molecules had been changed by a little bit. Or said another way, to achieve a certain viscosity, are 19 molecules of viscosity “A” plus 1 molecule of ultra high viscosity just as good a lubricant as 20 molecules of viscosity “A plus a little bit”? I have to believe that answer is no. Maybe the difference isn’t big, but I can’t believe it’s zero. And that, along with the much bigger and proven disadvantage of the VI improver shear problem, is why no oil with a spread greater than 25 will ever go into one of my cars until or unless somebody figures out how to make base oil with greater than 25 spread (or unless I started doing some track work, thus legitimately needing the wider spread, and was willing to change it much more often than my current personal guidelines stated earlier).