The whole lead additive thing is a bit hyped up to begin with anyway.
Back when I first got my P's, I bought my vk commodore powered by a trusty old 202, cast iron head, designed for leaded.
At first I ran the old girl on LRP, it was absolute trash, made the spark plugs foul up with a pinkish powdery substance, lacked power, etc.
At around 270000km I just gave up and filled the car with unleaded, I didnt even play with the timing, just drove it as was.
I removed the engine at 309000km to replace it with a fairly modified 6cyl and upon dismantling there was no real amount of wear and tear on the exhaust seats, we reringed the motor and put in new bearings and fitted it to a mates VH commodore, it went well for another year or more before it got written off.
Briggs engines are not really affected by super or unleaded, since the block is cast alloy they have hardened valve seats installed anyway.
