Hi!
Dar ...that is what I call carbon build up...!!
The picture you showed is a typical example of carbon build up inside the combustion chamber and I assume it's there on the piston sides as well.
This carbon build up is a problem when when you want the max from your engines, sport engines as well as other engines.
I agree it's not excessive but it's still there. I have seen much worse.
Using all synthetic oil or a mix of castor and synthetic oil prohibits this.

/Jan K