Hello!
Burned Castor oil (brown /Black residue) will always build up on piston top and sides and in combustion chamber when using Castor oil. This is perfectly normal.
When you run an engine lean you know that you do it because you HEAR IT! (This is a thing you will learn to recognize by time) And the engine stops or runs erratic.
You could also see that after a lean run the glowplug could be broken.
Never ever run an modelengine lean! This is easy to do when you know how to set an engine but for a novice it could be tough!
One way to get ride of the carbon built up is to run synthetic oil instead of castor....and this i what most sportflyers do nowadays.
Here in Sweden (and I would say all of Europe) 15% Aerosynt 2 or Aerosave from Graupner in Germany or the French brand Motul "micro"is what people use and these oils does not produce any carbon build what so ever (excess carbon on piston is no good). The engines are as new inside after several years running.
And they can stand up to heat as good as castor oil or at least what sportflyers demand of and oil , I'm not taking high-rewing 6,5cc F3D pylonracing engines here.......
As for marks (if they are large 0,1-1mm) on the pistonside...this is bad and tells you that the engine has inhaled some microskopic sand or grit..and this will give less power and the engine will perform less good in all respects.


Regards!
Jan K
Sweden