I used to switch oils without regard to what I had just drained.
Never had a problem with that.
I now prefer to use the same oil for at least a few changes.
Since you can usually load up on the FAR oil de Jour, this is no problem.
I have been doing UOAs on my little, mostly old, fleet, and it is probably better to do a UOA after a few runs of the same oil, if only to eliminate the leftovers from previous oils' add packs.
Brand loyalty?
Not really.
I do like Maxlife for the BMW, since I had a great UOA with it.
I like PP for most engines, since I've had good UOAs using it in the older Accord and the Impreza.
We'll see how much I like Synpower when I drain it and take a sample for a UOA next week.
My feeling is that if an oil has worked well in a given application as you use the vehicle, why not consider your oil choice dilemma solved?
OTOH, put a different API spec SL/SM/SN oil in the sump with every change and the engine will certainly still be running just fine when the vehicle is scrapped.