I don't comment much on oil threads, mostly do your thing if you feel strongly about it. Your bike isn't going to die if you don't change oil for 10k. Longevity of engine, well, don't sweat it, we're all doing just fine regardless of our preferences, no worries. It's not all that dirty, you do have a filter after all.
The transmission gears shear oil, but that doesn't degrade lubricating properties much, just shift quality. Certain oils will work better.
I'd stick with 10-40 mostly, maybe 15W40. 20 is getting a little too thick for those cold starts where you want oil quickly where it needs to go in the first 30 seconds.. Short trips bad, long trips good. If all short trips or commuting, then shorten your interval.
Oil will discolour with use, but it it's actually black, you may have a blowby problem, which a compression test will help determine.
Overfilling? I go to max when filling, I like my engines to have all the oil available. A couple ounces overfill isn't going to cause problems, you'd have to grossly overfill. Oil is well below the crankshaft, but there is the "windage" factor, so yes, don't overfill grossly.