There are lots of "theories" about this. In order for the auto makers and the oil companies to comply with milage and emission regulations engines and oil had to change. The roller tappet cams didn't need the zinc and since it made the oil stick to it's self as the parts it fought against better milage so it was removed or drastically reduced even in the diesel oils we depended on for several years. That is when the engine oil and auto tranny fluid turned up in standard transmissions.There are additives. The cheapest is good old STP. It contains enough zinc for a 5 qt. oil change. Zinc is not listed in the contents but I contacted the company about it.
The good news is that when reports of cam failures from cam companies became plentiful the oil makers took note and some things were chanced. There are posts on here about this but I don't know how to access them. Google it and separate the old horror stories. There is a lot of urban legend on this subject. Valvoline racing oil still has zinc. The additive called ZINC doesn't have any. The new oils probably have what it has.
