Hydraulic oil/fluid grade for suspension?

[azitizz]

Hi there, I need to top up my suspension hydraulic oil on an 85 300td wagon. I was hoping to flush it at the same time and replace the filter as the fluid in the big White/Beige container that says OEL on it is looking pretty dark. I read on a different post that it would be fairly easy to flush it simply by continuously adding the new oil to the container as you catch the old stuff in a different one. Im trying to find the right kind of fluid to replace it with. Does it make much difference or can you go with generic hydraulic fluid? Any advice on at least some specifications that should be considered when choosing the right hydraulic oil?
Thank you.
The lid seems to be leaking on the container and Im suspecting this is why I have to add more. It seems like the metal hose puts lots of uneven tension on the filler lid so it wont seal properly.