Do you have a multi cooler? Or a manicooler?
To my knowledge, it is true that the engines were not originally equipped with a vacuum-capable cap, just a pressure release. I don't think they operated at very high pressure, ~7lbs.
Personally, I don't see the harm in a recovery bottle. I've got them on my older 6.354(M).
If it seems to be going in and out, and when measured cold it is measuring the same every time, I'd be happy with it.
My memory is that those are filled to just below the filler cap neck. If you see the stuff underneath, it is too low. If it is up into the neck, it is too high. The real trick is to make sure there is no air in there, which requires filling slowly. I think there may even be a vent plug in the manicooler or multicooler or both, I don't remember.