First, do a search of the MANY previous threads on syn oil. There are as many opinions on synthetic vs. dinosaur lube as there a-holes. Second, your dealer is an idiot and/or a liar. How does he explain Syn 3, manufactured for H-D, heavily endorsed and marketed by H-D and prominently displayed at all H-D dealerships?Grayghost said:What's your opinion on using snythetic oil in a Twin Cam? Will it hurt the motor? My HD dealer said using snythetic oil would void my warranty.