I use whatever I have on hand for the most part. When I see a case of quarts of oil pop up on Marketplace I usually snatch it up regardless of what it is (brand, weight, or how old it is). I actually have 4 or 5 cases of old API SH Quaker State 10w30 I've been using it in my old Briggs flatheads and sleep fine at night. My Pathfinder has seen 0w20, 5w20, 5w30, 10w30 and 10w40 with no issues and no noticeable difference in performance between them.
Currently have Havoline, Pennzoil Full Syn, QS Full Syn, Castrol Syn and Fram Full Syn in my garage.
Always full syn but I don't care much about branding; as long as it meets specs I'm fine with it. I generally don't mix them unless I "have to." Never had any problems doing it, just think its an OCD thing or something???
Once asked my Dad when I was a kid, which kind of oil he liked. He replied "the kind that's on sale." He did all of his own maintenance and kept cars forever!