I have an issue. A few weeks ago, my 4th and 5th gears were slipping where my bike would rev like it was going into neutral. I saw another forum where it said to check and/or change the oil, which I did with full synthetic 10w40 motorcycle oil. The oil level is good, but now it is bucking and I have a crunching sound in each gear. This is an 03 Honda Shadow 750 ACE. Any advice will be appreciated.