A couple of thoughts on an "engine oil flush."
A):huh: I'm curious as to why is your engine is "so dirty" with under 20K miles? I have close to 40K on mine and I change my oil and filter at 5K intervals (full syn Rotella 5W40W/with Fram extended guard-silver 7317 filter). The oil that comes out looks "used" but it still has a hint of "honey color" left and definitely does not look "dirty." Do you regularly change your oil while the bike is cold or do you get it up to operating temp before you drain the oil? Warming it up gets gunk/contaminants suspended in the oil and then they are carried out of the engine when the oil is dumped. Perhaps you (or the PO) are/have been changing the oil cold? Just a guess.... And, as has been mentioned, it's a good idea to drain the oil thoroughly, if you are not already. I let mine drain 15-20 minutes after it has forcefully stopped flowing out the drain hole, leaning it side to side, back and forth. I don't know, I usually get around 1/4 a quart out that way after it stopped flowing/dripping forcefully....
B) I have heard stories (no first hand experience, so take this for what it's worth) from car mechanics who are solidly against oil flush products. They claim that it jacks up the engine seals...that engines that "get flushed" soon develop leaks from valve covers, main seals, etc....
C) Have you considered a product like Marvel's Mystery Oil (I know Risolene makes a similar product)? It's a "leave in" oil additive that, among other things, is supposed to contain detergents that clean your engine's internals. I do have some personal experience here. A while back, I had an old truck (Ford Ranger) that at about 250,000 miles developed ticking lifters. I changed the oil and added a quart of MMO (4 qts oil, 1 qt MMO). About 500 miles after the oil change, the lifters stopped ticking.... You can get the stuff most anywhere that sells motor oil (even Wally World). I have used it in some high mile motorcycles as a "just for drill, preventative measure" in the past with no problems. With my V, I will add 1/2 a quart of MMO at my 50K oil change, again, "just for drill."
:goodluck:
:cheers: