I’m not sure whether we disagree or not. It may just be semantics. As I mentioned back in post #11 by using the octopus analogy, VI improver additives do indeed cause the oil to behave like a heavier grade of oil at high temperatures. That seems almost identical to your ending statement that it behaves like a thicker oil when warm. But it is still way way way thinner when hot than when cold. For example, a straight 30 weight, single grade oil from the good old days has a viscosity of about 12 cSt (centistokes) at 212F and 2400 cSt at 32F. A modern 5W30 oil has the same 12cSt viscosity as the straight 30 weight at 212F, hence the linkage to the good old days oil by using 30 as the summer number. But it is only about 200 cSt at 32F rather than the 2400 of the straight 30 weight. The origin of the 5W in the designation 5W30 comes from the fact that a straight 5 weight oil from the good old days, just like the modern 5W30, would be about 200 cSt at 32F. But the straight 5 weight oil would only be about 6 cSt at 212F. So what has happened is that the combination of VI improver additives and use of synthetic base stock in a modern oil has made the viscosity change less with temperature than the old oils. But they still thin out as they heat up, just not as much.

As a bonus tidbit that was not mentioned in the earlier post, the biggest single advantage of full synthetics over dino oil is that they have a better raw VI than dino oil. You can get about a 25 spread (like 5W30) in a full synthetic without much, if any VI improver, and thus without the shear problem of VI improver additives. Unfortunately, if you go to an ultra wide spread like the 40 spread of a 0W40, you need VI improver additives even with a full synthetic, thus exposing yourself to the risk of shear even with a full synthetic. But tying back to the prior paragraph, even an ultra wide spread oil like 0W40 is still a lot thinner at 212F than at 32F. The designation 0W40 simply means that at 32F, the viscosity is the same as a good old days, straight 0 weight, while at 212F, it’s the same as the good old days 40 weight. The numbers if you are interested are about 18 cSt at 212F and 80 cSt at 32F. The reason for all the “abouts” in the writeup is that there is a fair amount of overlap in the grades, so the exact numerical viscosity in cSt depends on how your eyeball reads the somewhat fuzzy charts.

 

Remember Oil pressure goes down the hotter the oil gets. That wouldn’t happen if the actual viscosity/thickness was going up.