Yeah. It's the valve train right as the different thermal expansions give it it's greatest clearances. As it heats up some more the clearances close back up.
I hate doing valve adjustments. So in my never ending quest to use modern technology to validate my laziness, I've been running the Liquid Moly lube in the engine since new. It's been about as abused as an engine can be, short of rev limiter burnouts. Done a lot of hot slow running 2 up on forest tracks in 35 ~ 40 degree C. A lot of "trail bike" stuff. Way outside the design brief.
So far so good...right?