EP 75/80 is good for those boxes. EP 0 will do too they are agricultural by modern standards, so don't worry too much.
But as has already been said, if you've got the 5 speed gearbox, then ignore the 'max' mark on the dipstick, and fill it higher, to a point halfway between that 'max' mark, and the point where the dipstick meets it's head.
That adds enough more oil to overcome the insufficient splashover from the main box to satellite chamber that was a problem with these boxes when they added the bolted-on extra chamber that the 5th gear cog sits in.
It's useful to scribe around the gearbox dipstick with a hacksaw at that higher fill level point, so you know where to fill up to in future.