My setup/issue

So here's my problem. I have a b7 block with a z1 head( I
Assuming its a z1 head since I tried a vx head gasket and it hasn't leaked) the thing is though, I'm not 100% sure if it's a z1 or a z6 head. How can I tell the difference? And if it's a z6 head should I start using premium or keep using regular?
Sn: I bought the car like this so I didn't even know it had a different head on it until I put on a dx head gasket and it was leaking bad
Assuming its a z1 head since I tried a vx head gasket and it hasn't leaked) the thing is though, I'm not 100% sure if it's a z1 or a z6 head. How can I tell the difference? And if it's a z6 head should I start using premium or keep using regular?
Sn: I bought the car like this so I didn't even know it had a different head on it until I put on a dx head gasket and it was leaking bad