Sumping is when the suction side of the oil pump does not remove enough oil from the crankcase. When the flywheels rotate in the oil they turn it into a aerated state. This product gets introduced into the PCV hose that connects into the intake manifold. Once an abundance of this product builds up it ends up running into the air cleaner and saturates it. When you are riding the droplets of this oil hits the air rushing by and splatters down the right side of your shiny motorcycle. If you take a paper towel and wipe the underside of your aircleaner and it shows any signs of liquid, you probably are sumping. That's my less than technical version of "the World of Sumping". I have a 2011 CVO and had it happen to me at 17,000 mi. I corrected the issue with a different cam plate and pump (Fueling). I also did a cam change ( Tman) and lifter upgrade (S&S) at the same time. So far and 62,000 later everything is hanging in there. This is just one opinion of the many that are out there. If you search this site you can find many more technical explanations but I think this covers the basics. The bottom line is it sucks that it happens but there is light at the end of the tunnel.
This is accurate to a certain extent with older bikes. But the OP has a 2017 CVO. I’ve had 2 engines fail due to sumping on my 2017 CVO Limited. There was never a drop of oil that came out of the air cleaner. The sumping experience in the M8 is due to a miscalibration between the oil entering the crankcase and the oil being removed from the crankcase due to a defective oil pump and other engine design flaws. It is not an “all of a sudden” event. If you never ride more than 20-30 mins at a time, you'll likely never experience it. In fact you'll mask it because after building up oil in the crankcase, the next time you start the bike and let it idle for abit, it will empty the crankcase during idle. Sumping happens over time but only under certain circumstances such as rapid rpm changes over a period of time during hard riding in the twisties OR traveling longer distances at higher speeds on the highway. The amount of time is variable based on how mis-calibrated the oil flow is.
At first a sumping bike just feels slightly less powerful as it would on a normal hot day. But eventually the oil in the crankcase builds to the point that the crank is sloshing through the oil as it gets deeper in the crankcase. Eventually the crankcase fills leaving massive rotational resistance. Efforts to accelerate are met with...no response. The engine simply can’t rapidly change rotational speed because of the resistance.
Engine failure comes from there being so much oil build up in the crankcase, that the pan is starved leaving no oil to be pumped to the top end. The engine also overheats just from all the resistance of too much oil in the crankcase. The engine begins to overheat and components such as bearings, lifters and worse begin to fail. The rider will never see an oil light which is quite disturbing as a significant design flaw in my opinion.
If you suspect sumping in an M8 engine, you should immediately read the latest version (there are 7 versions) of HD's Service Bulletin 1450 (attached below)
I hope that helps.