[Old Tele man]

...unlike in art, a patina like that is definitely NOT something you want.

...some soaking in MacKay's Engine Cleaner will remove that gunk, but it'll also make all "wear" clearances/tolerances get bigger.

...lastly, the "top" compression ring might be a problem, because it rides right up against the "wear ring" in the cylinder -- installing a new ring with its new, unworn, 'square' ring edge will be "crunching" up against "the rounded" wear ring in the cylinder...and could result in a broken ring. MORAL: the top ring and cylinder bore cleanup are critical to longivity.