I gotta call BS on that one. 25 years in car dealership shops and I've never even heard mention of a viscosity checker, let alone used one.
I have seen the factory fill oil have a UV additive so they can tell if it had the initial oil change. I have also seen factory oil filters that are different than any on the market for the same reason. And I have known of the factory to request a sample of the oil, and also fuel in the case of diesels, for analysis.
So this does happen.
Best to use the right stuff
Rod