Depends on how much is leaking out
The shop that serviced my 964 that I just got it recommends Redline 20/50. 20 years ago, when I had a different 964 I used Mobil 1, and I don't think it has any zinc.
I was of the understanding that an engine that leaks at all will leak more with synthetic. Synthetic is more stable, and in most all cases a superior lubricant, and my understanding is that it precludes the need for zinc in the oil.
Some old Pano articles on this suggested that for those who want to use conventional oil,
the recommendation was for Shell Rotella which is for diesel engines, and has zinc. Further reading elsewhere suggests that Rotella isn't a good choice for gas engines, but I have used it.
It's my understanding that if the oil wasn't formulated with ZDDP, adding a ZDDP additives was useless.
Here's an older article I found some time ago:
https://www.jalopyjournal.com/forum/threads...engines.138386/