Lucas Response To Zinc Additive
ACCCC recommends the use of
Heavy Duty Engine Oils in older vehicles. There is no need to add ZDDP to a properly formulated engine oil.
There is no need to fear SM oils. The Starburst (ILSAC GF-4) rating is what specifies 600-800 ppm of phosphorus. CJ-4/SM HDEOs typically have 1000-1200 ppm of phosphorus, which is plenty for flat tappet valve trains. If you need more, the 15W-40 CI-4/SL oils typically have 1300-1400 ppm.
ACCCC Engine Oil Article