I've spent many a sleepless night worrying about mixing.
If mixing oil is bad, then changing to another oil is also bad (but less so).
And if that is bad, then even using the same oil is bad because formulations change.
In which case, the right thing to do would be to buy all your oil for the lifetime of your car from the same batch. But then you'd have to invest in non reactive containers, temperature control, vacuum sealing and automatic stirring.
So my next car will be a Tesla.