I've been mixing oil for 40 years! Today, I just do it differently.
1st, I did it out of need, as I didn't have much money and I'd mix all oils together for an OCI often without changing the filter.
I have mixed 5 different brands all being a different weight.
I wasn't trying to out think the chemist who have designed the oils, just trying to keep my car running.
2nd...Fast forward to today, I mix differently:
I mix syn and dino at something like...2 qts syn/3 qts dino OR...I will ad an orphan qt or two, to the other oils in my stash to either balance out the stash, or finish up a particular oil/weight/brand, mostly dino oils.
3rd...other example is:
I'll mix 3 qts 5W20 with 2 qts 10W40, if this is what I have in my stash. Though, I always try to buy oils that meet all 4 of our vehicles recomendations. 5W30 covers 3 of 4 cars, temp dependant but, each OM mentions different grades(except one car).
4th...I often buy up a store shelf of discounted/discontinued oil(all held in high regard here at Bob's) or, use sales/rebates, you get the idea? Any oil from, 5W20, 5W30, 10W30 and sometimes 10W40/15W40 though not often. And I have no reason to buy 20W50. So, my stash can often rival the best
No, I don't want to see your pictures!
I have never had an oil related engine problem in all of those years. I have removed the valve covers and the oil pan(to eliminate leaks) as many of these vehicles(too many to list) have had several hundred thousand miles on'em. PERFECTLY CLEAN!
Mixing oil may not be optimum but, it certainly won't hurt anything IMHO! As long as the OCI's are reasonable and the oils are for motor vehicles. I have always used/mixed oils that are for my engines though, I have used many diesel motor oils in gasoline engines and, that's a good thing
And yes, I use one brand/grade/wgt often! Lately, I only mix orphans just to use'em up, again, mostly dino oils!
Would I buy a new BMW, Lexus or Mercedes' and just start mixing oils? NO, not really! If I do though, it'll be a couple different qts of synthetic oil that I'd have in my stash that meet their requirements.