shear

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Iirc you can make a Full synthetic 5W-20 with no viscosity index improvers so mechanical shearing won't happen in that case.
If the oil goes down in viscosity it will be due to things like fuel dilution.
 
Before, when I was using only 5w30 oils for my Sienna (that was what it was originally specced for, although it was back specced to 5w20 & also 0w20) I gave that some thought. I looked at quite a few UOAs of 5w20 oils and it was very hard to see shearing. I don't recall the specifics, but in the UOAs I gathered, the oils either didn't shear out of 20 grade, or maybe one did, and it was borderline. This was one of the points that led me to use 5w20 in my cars.

If a 20 weight oil would shear, it would shear to the new 16 grade. I forget the requirements for both 16 & 20, someone here can probably chime in with that info.
 
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distinctions must be made as most synthetics regardless of their names or advertising is NOT a real synthetic that is less likely to shear. its stated a PAO 30W meets the 10W30 spec WITHOUT viscosity improvers. most all synthetics in most countries are predominately if not all group III base oils refined from CRUDE oil or even natural gas + are not up to PAO + or Ester standards + capabilities.
 
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