Pulled the plug out today just to check as it has a bit of an oil leak which is showing oil on the cap, inside and out. It has the copper washer like a glo plug. How best to seal it?

I also noticed the plug shows the engine is running rich and has a wet look and slight carbon inside plug. How should the plug look with an small ABC engine, light grey like other petrol engines? I dont want to go too lean and damage the engine

Regards
TtT