I read somewhere here that diesel oil is OK to use in these engines. Is that 15W40? And is it true? I have a '99 Tour Deluxe and it's oil change time. I thought maybe 15-40 would get rid of the clutch whine seeing as it's thicker oil. If not, I plan to use synthetic. Bike has 31,000 miles on it. Any help appreciated. Thanks in advance.