Its not the "CAR" oils that cause the Issue.Its that the Sythetic Oils do not like Wet CLutches. I have not seen one Manufacturer say that its OK to run Synthetic in a bike with a wet Clutch. I have alwyas been told and READ in the manuals , specifically NOT to use synthetic oil in a bike with a wet cluthch...so I dont....and I have no issues. I Use Mobil Clean 5000 in most of my bikes with good success over the years.