http://www.viscopedia.com/viscosity-tables/substances/engine-oil/
Pretty sure if it's SAE rated it does have to be a certain viscosity. I think that prior to standardization the viscosity may have varied.
So, as it turns out, the "SAE weight" is a rather broad range of actual viscosity. The better measure is centiStokes@100ºC (or 40ºC). With fork oils, the spread of actual viscosity within a particular SAE designation is quite broad (overlapping, even).
For example, motor oils marketed for "high mileage" engines typically are spec'd into the higher viscosity in the stated SAE "weight" range.
https://www.bobistheoilguy.com/viscosity-charts/
I wouldn’t consider that a huge range. Seems reasonable.
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