As far as viscosity at specific temperature goes, that can be calculated or graphed. You just need the viscosity at 40 and 100 C which almost all manufacturers provide on-line if you hunt. This is also a way to compare specific oils, say a brand X PAO synthetic vs. its conventional, vs a Brand B semisynthetic.
Viscosity Graphing page
Operational Viscosity

As far as how far they fall off in use, that's a good point and will require sending a sample of used oil to a testing company.
Originally Posted By madscientist
Originally Posted By 440Jim
Originally Posted By dvw
As far as vicosity vs temop. Drag motors don't gain a lot of oil temp. Mine seldom exceeds 160 with a diaper.
Doug
I was going to say something similar for drag race applications. I typically saw between 130-160F oil temp. And some days it took serious engine warm up to get the oil up to 130F.
At 140F, I am thinking a 10W-30 would be about 90 SUS.
At 200F, I am thinking a 20W-50 would be about 100 SUS.
IMO, Pretty close at associated operating temperatures.
And as previously posted, specific brands can vary from each other....



Your temp assumptions are correct, if the oil was built in the middle of the grade, or the high end.

What if you bought a 10w30 and the oil, out of the can, was on the low end for a 30? At temp it is out of grade.

IIRC, to pass the API grade, and oil only has to pass the test 5 times out of 10, again, IIRC. They don't have to pass 100% of the tests.
...

PS. Brad - Thanks for posting that!