Synthetic is not a problem but be sure you use a good quality motorcycle oil like Amsoil or Mobil 1. Motorcycle oils do not have friction modifiers like car oils do. Those friction modifiers are bad for the clutch. That being said the clutch is not your problem, sounds like you may have a starter issue now. Bad sprag clutch or broken boss allowing the starter intermediate gear to move out of place. The latter is a major problem, can be fixed but not for the homeowner to do, you need someone skilled to make that repair. It is not a part to be replaced but the case is broken if that boss is broken.

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