GTX is the one for the old Zeds. I changed once to GPS and it started leaking and the clutch slipped. Phoned up the Castrol technical department and they said stick with GTX if the engine likes it. Also, said that with modern oils it don't matter if you mix them about, not that I do, but once I was stuck out in the wilds and needed a top up, so I stuck some old stuff in and it was alright. I do use semi synthetic in my VN1500 Drifter because it gets up the top a bit quicker and the bike likes it. I know this because the bike told me. You have to change it regular and talk to the bike, otherwise they get grumpy.
Rod