Rislone (now part of Bars) has a ZDDP product for engines built prior to 2005 (diesels prior to 2007). As far as I know this is the first established supplier to sell an additive. Roller tappet Packards probably will not see much benefit, but for the Junior motors the company claims reduced wear.
While Mobil oil claims their products are universal using a product from a company like Rislone can't be a bad thing.
As for regular Rislone I recently had a ring sticking problem in a worn out engine. After adding Rislone the compression did come up enough for the vehicle to run better. So their products do work.