There is nothing to break in. It is simply recommended to wait 100-300 hours before calibrating to let it "settle". But there is actually no end to that settling either. If you want to maintain a perfect image you have to keep re-calibrating over the life of the display. This holds true for all display technologies.
I am not aware of any benefit. Most/all folks just watch them normally. I have heard there was a benefit to breaking in plasma's...in the olden days...different tech. Perhaps that is where the belief stems from. But Oleds...not heard of any, beyond waiting till there was over 100ish hours before doing a calibration...if you are planning to that.
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Yes as they shift so much in the start brightness on my C9 had to be at 56 to make AVS Black crush pattern to slight Flash as 17 now its 50...
Same experience here. I went from 54 to 50 after a few months of usage. I first thought I just got it wrong using my Spears & Munsil 4K disc, but I knew in my heart there was no way I was 4 clicks off.
So for those of you that setup your TV when you first got it, it's definitely worth putting that disc in again.
My C8 also started at 54 brightness when brand new and eventually went to 50. Definitely worth re-checking those test patterns every once in a while. And absolutely no question that you should wait a certain number of hours before paying for a calibration.