Very nice model. Just want to add that sometimes the smoothing errors can appear because of non connected vertices. So always check if everything is properly merged. Keep up the good work. o/
When you made the groove by cutting it out manually, the surrounding vertices of the groove were moved to a different angle. Thus the reason for the smoothing/shading errors. There are a couple of things you can do, all of which are trial by error: Select the vertices individually and move them manually so they match the…
It's different for each model, because the topology affects the smoothing. You need to experiment. james01 made several good suggestions, the third one is probably what's needed in your case. You may be able to get away with less vertices/edges than a full loop. We have some pics and explanations here.…