The hydro oil should be the same but....
In some older JD hydros, it was ok to use type F auto transmission fluid in place of the regular hydro fluid. Now they pretty much specify to use the low viscosity hydro fluid.
Flushing the cylinders might not be a bad idea. Not only because of the possibility of different fluid, but mostly because of dirt. In an older implement like your blade that has been taken on and off a lot in the past, could probably have some dirty fluid in it.
I don't think a "flush" is needed, but I'm sure it wouldn't hurt to at least drain the fluid from the cylinders and hoses. Then clean the fittings up putting it back together. I would just unhook the hoses and push/pull the cylinders to get out what fluid you can. Then when you attach it to you tractor, cycle the controls to fill the cylinders/hoses. Then bring your hydro level on the tractor back up to where it should be.