I will unabashedly steal this from a friend of mine who is quite experienced in mopars and automotive technology in general. I had asked him the same question since I was rebuilding my head recently. Dan Stern writes: "The mechanism by which Lead prevented valve seat recession is commonly misunderstood. It's nothing to do with lubricating anything. With an unhardened exhaust valve and seat, the valve and seat can micro-weld to each other if they get hot enough. Lead acts as a buffer to prevent this happening. The important thing is that exhaust valve and seat recession ONLY takes place when the valve gets hot enough to undergo localised welding. Then, when the valve opens next, the metal pulls apart like taffy. This roughens the meeting surfaces, and they become quite abrasive. The pounding/turning of a valve with such "pulled" metal on it creates a nice grinding wheel effect on the seat. In addition, the roughened surfaces no longer seal against each other properly, which eventually allows still-burning combustion gases to flow through the "closed" valve, causing a blowtorch effect on the poor valve and depriving it of any prayer of a chance to cool while it's on the seat. The blowtorch effect rapidly deteriorates the seal further, snowballing the seat recession. The main thing to remember is that this bad stuff *cannot* happen If the valve never reaches the crucial temperature. Whether the valve reaches the crucial temperature depends mainly on how the car is driven and used (Towing, drag racing or pedal-on-the-floor hauling WILL heat the valves--driving down the highway at a constant 120 klicks won't, and neither will hopping from traffic light to traffic light in the city or running down to the local grocery for a carton of ice cream.) Other factors in the margin of safety include the size of the exhaust valve, its material, and the efficiency of valve seat cooling in that particular engine design. The Slant-6 has ample exhaust valve seat cooling and stout valve material, and the valve itself is small enough relative to the combustion chamber area that you really have to abuse the engine before things heat up to the danger point. Very *VERY* little lead is required to prevent the localised welding and "taffy pull apart" effect that leads to the abrasive surface which, through incidental or positive rotation of the valve, eventually grinds-down an unhardened seat. The majority of the Lead was in the fuel as an octane booster, that's all. It was widely used because it was a very cheap and very effective octane booster. When unleaded fuels were first widely introduced (which introduction was brought about by legislation) , there was generally only one grade of unleaded available, and the octane was *quite* low--less than that of leaded regular. We all know that when you use a fuel of insufficient octane, your engine pings (detonation, pinking, pinging, spark knock--call it what you will.) This phenomenon creates *tremendous* heat in the combustion chamber--certainly enough heat to push the exhaust valves to the crucial temperature. Because for quite a while only unleaded fuel of subregular octane was available, plenty of people experienced these effects from using unleaded. While many of those engines that suffered under this low-octane unleaded really *DID* need the lead (high load and/or high-RPM engines), the bulk of the failures were due to the low octane increasing combustion chamber temperatures (see above). And so the myth was born that old cars' engines "WILL DIE" if run on unleaded. These days we have wider availability of high-octane unleaded fuels, which obviate the insufficient-octane cause of valve heating and subsequent localised welding. if you have an old car that is a low-stress application , used in daily-driver service, then you need have no qualms about using whichever octane grade of lead-free fuel your car runs well on and drive it for a Loooooooonnnnnng time with nary a valve or seat problem. Many US-based 6 and 8 cylinder engines fall into this category in normal daily driving service. The way to eliminate even the *possibility* of valve heating causing localized welding and subsequent seat recession is to install hardened exhaust valve seat inserts and exhaust valves of upgraded material (typically 21-4N stainless instead of 21-2N). This is utterly standard practice in the rebuilding of cylinder heads, and has been for years, as for instance when doing a gas conversion. Hard seats and valves are readily available for just about anything you want to put them in. It's a very common operation and a competent machine shop can handle it. But the main thing here is that there's absolutely no reason to tear into the engine solely to install hard seats. There is no collateral damage from seat recession. Drive and enjoy! You likely won't experience any problems for a LONG time, if ever. If you ever do, have a head job done. "