it won’t use more battery than a notification led.
If the screen has 60hz or higher refresh, I’m pretty sure it will. The screen itself may not use much, but the DAC will still use power.
I haven’t seen this actually tested, but many claim the difference in battery life is noticeable. I don’t think it matters much what app you use, many phones come with an AOD app, and I seriously doubt a third party app is better.
radix