Long term, 100K+ miles - and the difference between running Leaded and Unleaded fuel may start to show up. When it does, it still won't be a significant difference in a normal street engine. The lead additives in gasoline, in addition to being a cheap way to increase octane ratings, also had the side benefit of supplying additional lubricating to the valve stem/seal interface and the valve/valve seat interface. As lead was removed from the gasoline, the material composition of the valve stem seals, the valves and valve seats was also changed.. So the net effect in that area was minimal. I think the article you referenced - intentionally used "relative" terms like "soft" and "hard" because in order to be specific - they would have to have used a very large matrix. A matrix outlining make/model/year produced as well as each model using different engines - then showing all manor of copper alloys and steel... In general categories valve seats moved from copper alloy{other than tin}, to brass, to bronze, to steel, to hardened steel - as well as all of them with special alloy formulas between. The Bronze valve seats in the 71 were quite a bit harder than the ones used for years before that.. Ultimately of course the L28's started coming with steel. Bottom line - you may want to use a higher octane rated unleaded fuel, as they contain additives to replace the lubricating functions that lead used to add. FWIW, Carl B.