http://www.penriteoil.com.au/index.phpThere are brands of oil available here that contain zinc. I didn't use any oil additives in my Grand Ville (or any other Pontiac I've owned).
As far as the lead additive for gasoline, I've always thought that it was snake oil. I had a '64 Bonneville for 10 years that I used daily and ran it on propane (lpg). That car travelled 200,000 miles in the time I had it and I don't know how many miles it travelled before I got it.
When I finally pulled the engine, the heads were fine. They were the original and correct 389 heads. You could have serviced them and used them again.
A former GM engineer in cylinder head design, John Sawruk, visited Australia a couple of times. He told me many years ago that removing lead from gasoline was not seen as a major factor in valve seat recession. The main contributor to valve seat recession was the increasing lean air/fuel mixtures that the factory was forced to use to meet emissions. This was also the reason that the HEI ignition was introduced, as those lean mixtures were hard to fire with a points ignition.