One thing that I know you should keep in mind when you blend oils is that the physical properties of the individual molecules will not change much.
If you blend an oil that has a phase transfer of turning into a wax at a certain temperature (lets say 20 F for example) with an oil that has a phase transfer of turning into a wax at a lower temperature (lets say - 50 F for example) the molecules of the oil that change into a wax at the higher temperature will still change into the wax at or very close to the same temperature. So as an example in this mix, from 20 F to -49 F you will have a slush of wax molecules and liquid molecules.
In other words the cold performance of the individual molecules does not change, though you MAY end up with a slush that is still pumpable compared to a wax that will not pump if the oil that changed state at the higher temperature were the only oil used.
Also only pure oils change phase at exactly one temperature. Most oils are not a pure oil of only carbon molecules of only the same length, and each individual molecule chain length has a specific temperature that it changes phase at.
And also when you mix oils of different viscosity (AT TEMPERATURES THAT DO NOT CAUSE A PHASE CHANGE TO ANY OF THE MOLECULES OF THE INDIVIDUAL OIL MOLECULES OF THE MIX ) you end up with an oil close to the average of the two weighed with respect to the concentration of each, but not exactly that number. There are formulas for figuring out what that number is, but I do not know them.
And as ABN pointed out, there is some concern in some cases about the additives interacting.