I would have to run compression test and see if its loosing too much into the crankcase. Reason being, they haven't always had PCV systems, only breathers on valve covers. Oily air coming out of those made a mess on the valve covers and potential fire hazard. PCV allows only inhale at the breather(s) and any pressure actually vacuumed out in another place.
If you do add a pcv valve you'll need to know whatever brand valve cover you have also has a baffle beneath where the PCV valve will be or raise the valve away from the cover to a inline configuration so it doesn't pull too much oil out.
Good luck