Exactly. The viscosity calculator will do that. When you 50/50 combine a 12 cSt with a 14 cSt the difference is insignificant (12.96 vs 13), but try some 100 cSt additive on a 30 weight and the blender will come up much lower than arithmetic mean.
If you have a decent calculator you can do it yourself. The blended viscosity is the antilog of the weighted averages of the logs of each oil mixed into the blend (or simply 10 raised to the "weighted averages of the logs of each oil" power.