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>>>Most say that a 0W-XX oil is a 0W oil that has been engineered not to thin more than XX when hot... (and a 15W-XX is a 15W oil that won't thin more than XX...)
That's quite oversimplified. Most (if not all) of the 0W-xx oils are made with synthetic or hydrocracked base stocks, which inherently act as multigrade.
The initial part, the W-, is a measure of the resistance to flow at cold temperatures. The higher number shows the resistance to flow of the oil at 210F. The non-synthetics were often created by using a light oil (which would give the 5W- or 10W-, and then adding viscosity index improvers to keep the oil from thinning out too much at high temperatures.
>>I also read about the "spread" being a factor as well... 0W-40 has a 40 spread, while 15W-50 only has 35, and 10W-30 only 20... The theory being that the wider the spread, the more additives and finagling has been put into the oil to make it perform like that. That's that much more stuff in the oil that is in there for a purpose other than lubrication.
The spread can be a factor. Synthetic base stocks can be chosen and blended to give much of this spread without any VI additives. Some years back, AMSOIL blended both 15W-40 and 10W-30 oils with no viscosity index improvers.
When non-shear VI improvers became available, the oil could be reformulated to be even better with virtually no shearing. The non-shear VI improvers are more expensive, and all oil companies may (more likely do) not use them.
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