As said, cos cossies have hardend valve seats they can run on either, obvoiusly before they ran on leaded but as soon as super etc came out they advanced the ignition to make it work on unleaded, but super or optimax is best or will run like a bag of shite, and cos super etc have a higher octane their better anyway

Dont think valve stem seals makes a difference as far as i know they've always been the same but could be wrong.

Thats why you have red top for the older ones (leaded originally) and green tops fro the later ones, althou that sometimes they varied from the factory they were assembled at.