I see in a number of topics that CI-4/SL HDEO are recommended to protect older gasoline engines with flat tappet valve trains, especially those with high lift cams and stiff valve springs. Widman suggests in his Corvair Flat Tappet Engine Oil article that a good CI-4 HDEO contains 1000-1200 ppm of phosphorus. He also suggests that phosphorus levels greater than 1400 ppm can lead to increased engine wear and deposits. Esso's Flat Tappet Engine Wear Bulletin suggests that only 300-500 ppm of phosphorus is required to protect (OEM-style?) flat tappet valve trains. From what I can see, the mainstream CJ-4/SM HDEOs still contain 1000-1200 ppm of phosphorus. It is the CI-4/SL oils with much higher levels of phosphorus (like many 1300+ ppm 15W-40 HDEOs) that appear to have been reformulated to have a maximum of 1200 ppm of phosphorus. How much ZDDP, or more specifically phosphorus, is enough for high-lift flat tappet camshafts with stiff valve springs? Does the actual amount of phosphorus really matter if CJ-4 HDEOs still have to meet the performance requirements of CI-4 HDEOs?