What is a "large amount" ? More than a few oz? Something very wrong there either way.
I'd open the drain plug, fill plug and remove the oil filter. The used compressed air to blow thru the system from both the filter and filler port.
IDK how hard it is to remove the oil pan, but that's something to look at too.
I'd get about 3 changes of oil (12 qts?) run 1 thru with just cranking at the start to let it move around, maybe while cranking, leave the filter off and let it come out there. Then with the filter on and then run for a bit and dump it out along with the filter. Move the bike around to get the oil to mix up and then dump it.
See how it smells / looks after that, then repeat. On the last one or two changes, run the bike until warm and move it from side to side to pick up sludge from the top.
Another option is to remove the valve cover and maybe run a cleaner thru it. IDK about SeaFoam or others stuff, but maybe worth a try.
As far as where it came from, I'd look at a dead cyl or EFI leak.