Trust me you want to get rid of it. Even if it's a thin "stain", it will act as a formation site for a hard fouling ring. I used several recommended oils in my patch lubes that caused this problem, for example the Napa water soluble oil where they changed the formula
. Loading would get difficult and accuracy suffered when the crud ring tore patches and roughness advanced up the bore. Scrubbing with green pads would smoothe it out, but it wasn't right until I took a wild guess that it was basically baked on grease and cleaned the barrel with heavy duty oven cleaner (the kind with lye). After it sat in the barrel for a few minutes, the first patch brought out a number of tiny slivers about 1/4" long and the width of the grooves. Scrubbed a few times with green pads and water, then cleaned and dried normally. Problem solved, accuracy and ease of loading restored. I have also used lye and water with similar results. Be careful with lye in any form!
In the case of the slivers, the bore was sparkling except at the place where the ball sits, where it was black, but a tight patch went down without any resistance, and the barrel would load and shoot normally for a few shots.
Also, I have become religious about wiping any oil out of bore before shooting and use only water as my patch lube for range use.