If you read page 14 of the article, you will see a synopsis of ZDDP in automotive motor oil. It's clear that API automotive spec does not care about the older cars. Read on, and you'll see that in truck fleets the turnover is only 10% so they make more effort to protect older engines. If CJ-4 was the same as CI-4+, they would not have come out with a new rating. They removed/reduced the ZDDP to meet new emission restrictions.

There are many different protective qualities, but most all sources agree that low ZDDP levels can cause the premature destruction of flat tappet cams.