Zddp

It was the auto manufacturers (ILSAC) NOT tree huggers who specified lower phosphorus levels in oils due to industry pressure to extend warranty periods on catalysts in particular, and engines in general. Thread from an oil forum http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1049812

Keep in mind your slant was designed in the late 50's before ZDDP was added to oil in the high quantities seen in the late 60's-70's. Modern API "SM" spec oils have about the same ZDDP levels (.08%) as the oils of the late 50's. Otherwise if you were to compare the best oils available then to oil now, well, you just can't buy oil that bad today.

Back in the 80's everyone said that unleaded gas our cars to blow up. Your car obviously survived. And now, despite the measurable increase in average IQ from reduced friable lead in our environment, people still insist on believing whatever ballyhoo some chemical marketeer tells them.