If you have a VOA of that exact batch of oil to compare KV100 and KV40, and you have accurate fuel and water dilution numbers from GC (ie: NOT Blackstone), then you can calculate a ballpark area of where the viscosity should be from dilution alone. Even then though, there's oxidative thickening to take into account. If an oil starts at 10.5 cSt, ends up at 9.0 cSt after 5,000 miles and 2.5% fuel dilution, then it would appear about half of that viscosity loss was from fuel and the other half was from permanent shear. However, the oil would've thickened from oxidation to that 9.0 cSt point. In reality, the oil was likely down around 8.0-8.5 cSt before thickening to 9.0 cSt. Even if an oil shows the same KV100 after 10,000 miles, the reality is that the oil likely fell -1.0 to -1.5 cSt with use and oxidative thickening brought it back up to the starting KV100. A high amount of soot in the oil will also increase the KV100. The margin of error of the viscometer is also a factor. There's a good bit that factors into understanding a given UOA result.