fork oil suggestions

Does anyone how much fork oil is needed for gpz550 forks and what kind you would recommend? I think the forks are from an 83 (has a forward axle and an air pressure equalizer tube).

I can't say how much fork oil your bike takes, but I HIGHLY recommend Bel-Ray Fork Oil. I've been using their 15W in my '77 KZ650C1 since 1978 (changing it every 2 years) and have never had any seal leaks or other issues. Different weights of their oil can be mixed together (like a combination of their 10W & 15W) so you can fine tune the damping characteristics if you like to do that sort of thing. Ed