Please go the website www.heavydutycycles.com and read about oils. Read in the technical articles section. There are a few good articles named....Lubrication and your Harley. Oil facts for your harley. SJ Oil controversy. And last motorcycle oil verses automotive oil.
Both bikes BMW and Harley are air cooled twins and should behave the same towards oil requirements. I do know that heat destroys. And an increased oil capacity using a larger oil pan will help drastically. This pan will sink heat and so will the added oil. A small oil cooler installed in series if possible is the best solution. When fossil oils reaches 250 degrees oxidation of the oil increases by 55%. Synthetics start to oxidize at 300 degrees farenheit.
John