Flat tappet engines only need 1110-1200 ppm of phosphorus (anti-wear component of ZDDP). See
CJ-4/SM HDEO & Flat Tappet Valve Train Protection. Excessive levels of phosphorus can lead to increased engine deposits (>1400 ppm) and cam spalling (>2000 ppm). Any good Heavy Duty Engine Oil (HDEO) has all of the ZDDP to protect flat tappet engines. If you're running very heavy valve springs, you can use an HDEO with more than 1200 ppm of phosphorus like the CI-4/SL 15W-40 oils.
Unless your engine oil is running very hot, a 30 weight HDEO (0W-30, 5W-30, 10W-30) should be fine. If ambient temperatures never drop below freezing, you can safely use a 15W-40 as well. Excessively thick oils (like 20W-50) just waste horsepower and fuel.
As for adding ZDDP to engine oil, are any of you calculating the amount of ZDDP to add for a final concentration of 1200 ppm? Or do you just dump it in and hope for the best?
ACCCC HDEO List
