There is no easy way to determine this, because different oils will shear differently. For instance, I've seen oil analysis results where 10w40 Castrol turns into a 30wt oil, so it's thinned out a lot, but I've also seen 5w30 Amsoil results where it's thickened up to a 40wt. And I've seen 5w30 Mobil 1 results that have thinned to a 20wt.
So there is no easy way to figure out what is going to happen. Generally speaking though, a 10w40 dino oil will thin out to a 10w30 rather quickly, same with a 5w30 dino, it will thin to a 5w20 too.
A 20w50 and a 10w30, synthetic or dino, will generally stay pretty stable because of the lesser need for VII in those viscosities.
The best thing to do is look through the UOA reports on here and see where the oil's final viscosity has ended up. It's one of the first things I look for when I see a UOA posted on here, is to see if the oil was still the same viscosity.
Dr. T, Too many formulation variables,
Shear stability measured by a combo of ASTM 445 and 5275 tests is good to determine what the bearing will experience as far as oil viscosity loss. Lower vis loss = a more stable oil.
ASTM 4683 is a good one to gage the operating vis a bearing will see at temp under stress. The higher the value exceeds the minimum the better the oil. SAE 30W 2.9 cP same for 40w for cars.
40w trucks = 3.7 cP