I had my ´04 RK Custom for about 5500 miles. Recently, I switched to HD synthetic and noticed slightly less vibration at speed. I´ve ridden 500 miles since. I have not noticed any additional engine noise. Valve ticking at freeway speeds is about where it was with non-syn oil. I have heard it is not recommended to switch back to non-syn. I don´t know the reason why however. I do know that my BMW automobile really liked Mobil 1 and I noticed a significant improvement. I ran Mobil 1 for 50,000 miles with virtually no compression or oil loss. But that´s not a applicable motorcycle issue, being air-cooled.