I wouldn't actually call it burn in. OLED works, like almost all displays, by using a red, a green and a blue subpixel. It's the blue subpixel that is at fault here. Due to the way OLED works, the blue organic polymer decays three times as fast as the other two. So, if you use a lot of blue, the blue will begin shifting color compared to the other subpixels, giving a sense of 'burn in'.
Depending on the screen brightness, and the colours most shown on the screen (especially white or pure blue) the effect can become more noticeable. Most screens just dim equally, so you won't notice it unless you view it in a pure dark room. Best thing to do is staying away from max brightness, setting it on half gives you twice the lifetime.
And if you know how to make blue OLEDs that have a long lifetime, a good efficiency and produce a deep bright blue, contact me, we can become millionaires.
