In the 4 & 6 cyl MG/Austin/Triumph engines I work on which run at much higher rev's than the FE's but also have flat tappets I follow the same theory as this fellow who has proved his info with "real world" mileage.
Run the ZDDP at break-in and possibly until the first oil change then change regularly with a good oil (with or without ZDDP) after that. At his writing of the article he had 75K on his engine. He has since retired and is crisscrossing the country working on folks cars just for fun. Probably double that mileage by now and he pulls a small trailer everywhere he goes so the little 4 cyl is always under load.
As for my first Harley it was a 1975 90cc enduro.....
........no I'm not kidding (wish I still had it).