Intereting, I have stayed away from syntheric, completely based on my experience with older motors and weeping from gaskets (just my personal experience).I use Rotella Dino 15-40 and did not notice a variance in shift quality. I have noticed that I can feel a slow deterioration in shift quality as the oil gets older. Generally I change it as soon as the shifting feel changes. This is usually after a out 250 miles of off road riding. I assume it is all the shifting and clutch use, low speed, high temps vs. on-road use.