Running for any length of time at higher rpm's may in fact play a part in the sumping issue, but that doesn't mean people should blame those who ride at 4000 rpm versus chugging along at 2200 like many Harley riders tend to do. The problem is fairly basic, there is more oil being pumped into the engine than is being returned to the oil pan, until the dry sump is overloaded with oil. The oil pumps are constant displacement units, so the faster you spin them the more oil they pump. Think about it. At 4000 rpm for instance, in theory the pump can pump twice as much oil as it can at 2000 rpm. If the return side of the system has a bottleneck that limits the amount of oil that can be sent back to the pan, and that bottleneck translates to only allowing enough flow to balance the supply side at 3000 rpm or less as an example, the guys running higher rpms for longer periods of time would be more likely to experience the problem. A properly designed and manufactured system should be able to handle maximum rpms for extended periods of time with no backup of oil in the system. If that wasn't the case, you wouldn't see dry sump engines in so many race cars. I think I remember suggesting way back at the beginning of this saga that the aces at H-D might want to consult with the folks from Porsche if they couldn't quickly figure out this sumping issue. Porsche doesn't have sumping problems running at more than double the rpms of Harley's engines. And Porsche designed the most trouble free engine Harley has ever produced, the Revolution, proving it can be done but obviously not by the folks working at Harley. Proving once again top quality engineering is much more important than just hanging your hat on styling for the past thirty years.
JMHO - Jerry