Originally Posted by old1
I won't need any for some time, as I have a few gallons of the old hddo oil from before anybody started to lower the ZDDP in it, but at some point I will have to start to use more modern oils. What I am wondering is if since my engine is a well broke in 65 200 ci stock engine, is it even necessary to use an oil with more than the SN amount of ZDDP?
For flat tappets you want a 10W-30 or 15W-40 -- in other words an oil with a thick base oil, as the flat tappets temporarily shear the VIIs, which, as a result, cannot thicken the oil as intended. Don't worry about ZDDP -- modern oils have a better antiwear package than oils of the past with high ZDDP.
Amsoil ACD 10W-30/SAE 30 is a monograde, and it's as good as it gets for a 10W-30 -- it has a thicker base oil than that of a 15W-40.