which oil to use

Some of the synthetic Amsoil formulations have some of the highest, useful concentrations of zinc and phosphorous. I've been using the stuff for many years.

As mentioned, the hardening seals is a fairytale.

Here is a link to an Amsoil Technical Bulletin on the products and the issues.
https://www.amsoil.com/dealer/techservicesbulletin/MotorOil/TSB MO-2007-08-08 Flat Tappet.pdf

Amsoil source & info site:
www.thelubepage.com