The valves arn't the problem it's the seats which if not hardened can lead to VSR (Valve seat recession) only way I can think to tell is to remove the head then see if there are hardened seats, TBH the unleaded thing is a bit over hyped as the engine has been run for many years on leaded this leaves a "Lead Coating" on the valves etc. If driving hard or towing etc, lots of motorway driving then you could well burn a seat out but under normal conditions it shouldn't have any impact.

You can use Lead additive if at all worried.

Martin