Synthetic all the way. I think... NOT 100% positive... but the only bad additive in some oils is called something like molybdenum.... I butchered that word. That additive makes your clutch slip. So find a synthetic without and run it. ALTHOUGH, if you've been running your bike for many hours with a regular style oil, you shouldn't switch to a synthetic. It'll blow it up. When you run an engine on conventional oil, all the parts wear into each other at a lower tolerance. Meaning that the fit up gets sloppy, to an extent. Sloppy is a bad way of putting it... but anyways... when you add a slippery synthetic oil to an engine that is already wore this way, the looser tolerances of the parts from running a conventional oil will make the engine rattle apart. Also, once you switch to a synthetic, you can't switch back. I can't remember off the top of my head why you shouldn't switch back, but I've always been told you can't.