I just bought a xb12s and switched to the same Amsoil 20W-50 and can tell that it idles a little smoother. I also changed out the primary oil for Amsoil too. I was trying to figure out what Synthetic Oil to use and there was a whole discussion on it on the XB forum after I also asked the same question. lots of good feedback, but ultimately the Amsoil won..Good luck with what ever oil you choose. No one here will steer you wrong! :D