Motorcycle engines tend to lead a much different life than the average car or truck. They often see hard use during the summer, and when the weather gets cold, many of them are stored for the winter.
Is the hard, seasonal use too much for your engine? Is your air-cooled V-Twin running hot and causing the oil to oxidize and thicken up? If it shares the oil with the transmission, is there any way of knowing if both systems are wearing properly?