The service manual for my 87 EVO engine Soft Tail reccomends 10W-40 dino oil, not synthetic. Since it had 87K miles on it when I bought it, I thought a slightly thicker "low" viscosity number would provide better cushioning for the parts. There was also a noticeable amount of oil getting into the air cleaner from the rocker box vent tubes, which improved a little with the switch to thicker oil. So far, it hasn't started "marking its territory" like most Harleys, so I check the oil regularly to be sure it's not empty!
Jerry
