Does anyone use ZDDP or worry about the wear factor when it is not used?
Is there enough zinc in the ZDDP to foul a Model T Magneto with continued use?
ZDDP is a liquid, not a metal, found in motor oil in quantities measured in parts per million. It is not needed in a Model T, but it won't hurt anything. ZDDP started to be used in motor oil when President Eisenhower was beginning his second term. By that time the youngest Model T was 25 years old, and had never used ZDDP.
ZDDP was needed in high performance engines that started to become the norm in the mid 1950's to prevent scuffing on camshafts and lifters. This problem was brought about when valve spring pressures started to reach 100 pounds closed / 300 pounds open. These sort of spring pressures were needed to let engines rev above 5000 RPM, in order to reach then unheard of levels of power near 300 HP.
A Model T valve spring by comparison has about 5 pounds of pressure closed, and maybe 10 pounds open, in order to support 1700 RPM, and 20 horsepower. ZDDP is simply a don't care for our cars.
I agree with Royce. My 1971 Plymouth GTX with a 440 cubic inch V8 seems to run just fine on modern motor oils without ZDDP, and its spring pressures are at least 35 times higher than what Royce quotes for a Model T.
Admittedly, when we first broke in the camshaft in 2001, we added a bottle of GM "Engine oil supplement", which was rich in ZDDP. That first fill of oil was dumped, along with the filter, after the initial break-in and replaced with regular motor oil.