Why isn't all oil 0W-xx

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Someone please educate me. After months of research, I still don't understand oil weight and viscosity.

My main question is if 0W-20 and 5W-20 perform the same at operating temperature and most engine wear occurs at startup, why use 5W?
some random thoughts- not necessarily facts:

- If you drive hard, most wear occurs at WOT/ mid- high rpm and load

- I can think of only one ILSAC 0W30, there are more 0W20 due to CAFE

- Initially Asian 0W20 (Idemitsu/ Subaru) typically were at the low end of the KV100/KV40 vs Dept Store offerings
e.g: Current VW508 from MOTUL is PDS reads 38 / 7.8 mm2/s for KV40/100

- Most live in a climate that would never require a 0W rating
 
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Most wear does not occur at startup. The momentary wear rate, for that 1-2 seconds, may be higher, but the wear accrued is minuscule compared to the total wear the engine sees throughout its life. Whether the oil is a 0W-xx or 5W-xx is likely to have little to no effect on startup wear unless the temperature is too cold for the oil to pump effectively.

The issue with 0W-xx oils is volatility and shear stability. A 0W-20 will (typically) have a lower viscosity base oil blended with a higher amount of viscosity index improver (VII) polymer to bring the KV100 up to a 20 grade (or 30 or 40 or whatever). Lower viscosity base oils tend to be more volatile meaning they evaporate at a higher rate in very hot environments such as seen around the pistons and rings. Oils that are more volatile tend to leave more deposits around the rings, leading to ring coking/sticking, and sometimes more oil consumption due to evaporation. A higher amount of VII means the oil is more prone to permanent shear causing a drop in viscosity with use.

Therefore, if you don't need the extra cold startup protection, you're better off to use a 5W-xx or 10W-xx oil (in most cases) that may be less volatile and more shear stable. Note this is not accurate for every brand and grade. Some brand's 10W-30 may be more volatile than another's 5W-30 or 0W-30. What's stated here is the general trend. It's important to research the oil you wish to use to ensure it fits the needs of your engine.
 
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Agreed. The whole notion that a particular winter rating makes an oil “weak” (whatever that means exactly) is silly. Oils are formulated differently and carry different licenses, specifications and different approvals. That’s what determines performance not some blanket statement about the winter rating.
Apples to apples it is fundamentally weaker to shear. Demonstrated over thousands of UOAs on this very forum, and true for all multigrades.
 
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