How often does anyone change their oil in this machine? If I read the book correctly it read that the oil should be changed at 600 miles then at 7500 miles. Does the oil hold up for that extended period of time and mileage? I have had an air cooled four cylinder half liter rocket in the 80's and would change the oil every 1500 miles. I have performed the 600 mile change and I have 1200 miles on it now, would it be a waste to change the oil at 1500 or should I go the 7500 miles. Can the oil hold up that long? I use castrol gtx10/40 for the winter and 20/50 for the summer months!!! Any input would be great. Thanks and safe and fun riding.