Another issue you need to be aware of is that modern engine oil is not just oil - oils use many and varied additives to achieve specific ends. Detergents are used to remove contaminants from engine components but there are many other additives that help maintain viscosity over the life of the oil, and yet others that help the oil deal with extreme heat and/or cold and lots of other things.
When you mix different brands of oil or even different types of oil from the same manufacturer you risk the various additives working against each other and the oil delivering less than optimal protection.
The science behind modern lubes is almost "rocket science" and unless you really know what you are doing I think it's better to follow the expert's recommendations.
The only reason I would mix them is in an emergency. There are way too many variables to be certain. Sure there have been semi-synthetics for years but they were formulated that way. One variable with your setup will completely change the oil.
Now that being said, I work in an oil refinery with massive rotating equipment. Said equipment runs 24/7/365, some for years at a time without shutting down. We recently started upgrading all our lube oil to royal purple. In the equipment that could be shut down, it was swapped out completely but reservoirs were not cleaned or anything. On equipment that couldn't be shut down, we would drain a barrel of the old and add a barrel of the new, one at a time, until our rotating engineers thought the ratio of old to new was sufficient and said to stop. Keep in mind, some of the crankcases on our bigger equipment hold 12+ BARRELS of oil.
I have also read of people mixing their own but I have a hard time overriding the engineering behind multimillion/billion dollar corporations.
Watch Project Farm on YouTube and see the oils he tests. Pretty amazing test he does, excellent video. Not just oils but he tests glues, duct tape, bed liners. Myself I run Amsoil 100 percent synthetic which has been in my vehicles since 1976, when it wasn’t cool to run synthetics. Everyone would say it would leak out and wreck the seals.
Then Mobil 1 came along and it was suddenly it was cool stuff.