2019 GT and Break-In MPG

[collegekid]

I've been on Fuelly for a while with a few different older vehicles, but this is my first new one. I'm surprised at how efficient V8 engines have become. My 88 5.0 was 20 mpg average if I drove like a granny, and I can get almost 24 with double the hp on my 19.
I use regular unleaded and drive it like a maniac half the time.

Since this is my first new car, I wonder about something a mechanic friend told me. After your car breaks in (usually about 10k miles) your mpg will drop noticeably. Is this true? The mpg I'm getting now is great and I would hate for it to start dropping.