big,
The link you've provided does not directly address engine oils.
However, I tend to agree that mixing oils does not produce an optimal result, and should be avoided in anything not thought of as a beater.
I avoid mixing oils in my cars, and I stick to the same oil for at least a few changes in most cases.
The people who developed the finished motor oil know more than I do, or does almost anyone else on this site, and the basestock was carefully seleted along with the additive package to provide an oil that meets the applicable SAE grade and API spec to which it was blended, along with any manufacturer's standards claimed.
To think that I could produce a superior, or even equivalent, oil by blending oil A with some proportion of oil B is absurd.
The claim that all oils are required to be compatible by API spec is okay as far as it goes.
I could also mix tap water with a nice Italian mineral water, and still have a potable water, for example.
The claim that an engine sounds and feels fine on a given mix is also suspect.
You could put one of PQIA's worst performers in your engine, and it would sound and run fine, for at least a while.
Mixing may not cause apparent, imediate harm, but I personally avoid it in anything not given terminal beater status.