Valve stem seals are a critical part of rebuilding a cylinder head. The o-ring style seals are used on the early small bore heads. Later heads that have the modern valve guides with groves in them can take the umbrella style valve stem seals. They can be used on both intake and exhaust valves.
As an engine wears, the clearance between the valve stem and guide increases; as this gets beyond tolerances the car will tend to smoke under deceleration (when you take your foot off the throttle). Good valve stem seals will allow for greater wear before the smoke becomes intolerable. These were an improved material with little springs in the neck to help create a better seal than the original ADU4905 seal. But for the best available see LJQ101160 made in Dupont Viton material as used by Rover on the MPI engine.