Some swear by avgas some by mogas. Iam a mogaser (98 shell or bp doesnt seem to matter) at around 50 to 70c cheaper a ltr and cleaner burn its the better option in my opinion. Ive seen similar deposits to yours in our club aircraft. Even the old holden running on super was never that bad with all the oil it burnt
A local flying school did 1000 hrs on mogas and cylinders etc looked as good as new with as new compressions.
Nothing wrong with mogas as long as its not too old, 4 to 6 wks in my experience and you make sure your fuel lines dont induce a vapour lock by having high points along their route. I leave the electric fuel pump on all the time just in case.
Have heard of rings getting stuck in their groove from high lead deposits but no other problems that I know of. The octane of avgas is quite high.
Like most things each has its good and bad points to consider. As far as decoking goes, in my opinion, I wouldnt worry unless oil consumption / compression was an issue.