This should cover basics, including the fact that motor oil always get thinner when temperature rises. Despite of being very thin, 0W is thickest at cold startup. Therefore, if oil consumption increase and blow-by ever happen, they happen at operating or higher temperature when thinned out.
http://www.kewengineering.co.uk/Auto..._explained.htm
Also see this link, great discussion on oil shear, which leads me to the theory - 5W30 is only superior to 0W30 at operating temperature because of its less reliance on VII and thus the greater resistance to shear damage.
https://bobistheoilguy.com/forums/ub...t&Number=83298
I quote two very informative posts from that thread as below:
Quote:
'Shearing' (and changing hot-viscosity as a result) happens to oils that have viscosity-index improvers added. VIIs are 'strange animals' in that they thicken when hot, just the opposite of all the rest of engine-oil base stocks and additives. They do this by combining into longer molecules and decombining when cooled. However, the long molecules can be cut into parts and apparently these partial molecules don't recombine when hot as well as the original VII molecules do. So as more of these VII molecules get cut into pieces (ie, 'sheared'), the combined oil thins more than it did when new. That's why we talk about 'shearing down' to the next-thinner rating.
Synthetic base stocks have higher inherent viscosity indexes (ie resistance to thinning when hot) than dead-dino basestocks, so the synthetics have either no or at least less VIIs in them. For instance, Red Line's only oil to use VII is their 5W-40.
BTW, VIIs have very little of the other characteristics we value in oil--eg oiliness, film strength, detergent/dispersant, etc.
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Quote:
Viscosity Index Improvers (VII) are a polymer of which the molecules are much like a tangle of string. When they are cold the tangle shrinks, when they heat up the tangle expands.
To make a multi-viscosity engine oil, like an SAE 5W-30, you start with a thin "base" oil and add the VII to it.
When the oil is cold the VII shrinks and the thin oil flows easily passed the VII molecules. Viscosity is "resistance to flow" so in this case the oil flows easily so is regarded as thin at a low temperature such as - 25 deg C, eg. SAE 5W.
When the oil heats up, so does the VII and it expands. Now the thin oil (which has also thinned out as the temperature has increased) has difficulty passing the VII molecules, it flows slower, so it is described as being thicker, eg. SAE 30. However this means it is thicker at the high temperature (100 deg C) that engine oil is measured at than the non-VII treated oil would be.
Now VII molecules come in different sizes. Larger molecules are more effective at slowing oil flow, or thickening the oil. So less of a large molecule VII needs to be used to get the same multi-viscosity effect. You have to use more of a smaller molecule VII to get the same oil thickening effect.
BUT, and ain't there always a "but"! The larger VII molecules get "sheared" down easily, that is they get chopped up into smaller VII molecules, and then there isn't enough of them to provide the oil thickening effect. So what was an SAE 15W-40 made from a large molecule VII will become something like a SAE 5W-20 or worse. This is called "permanent shear", it is irreversible.
A multi-grade engine oil made from smaller molecule VIIs will cost more - more VII and it is more expensive - but will resist shear down.
Now where I've said "VII thickens the oil", you should realise that at low temperatures any oil is really thick. What a VII does is slow down the thinning of an oil as temperature rises. Comparing just the thin base oil with the thin base oil plus added VII, the base oil will be slightly thinner at low temperature (the VII will thicken the base oil slightly). As the temperature rises both oils thin out, just that the VII treated oil will thin out less over the same temperature rise, until at 100 deg C the VII treated oil will be considerably thicker than the base oil at 100 deg C.
Then there is "temporary shear". This is where a VII treated oil passes between two moving surfaces, the piston rings and the cylinder wall is a great example. In that narrow gap the VII molecule tends to get stretched out from a tangle into a line and its oil thickening effect is lessened. However when it gets out of that stretch zone, the VII moecule returns again to its tangled state and its oil thickening effect returns. Thats temporary VII shear.
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