Its Zinc Dialkyl-Dithio-Phosphate and it replaces what the Epa has regulated out of modern oil. The Epa has determined that the zddp in oil was contributing to the destruction of Catalytic converters and smog.

So now most of the oils have drastically cut back on the Zddp.
Zddp helps the break in on new cams and cam lobe and lifter wear over the life of the motor
We've had a few motors that have come back for a freshen up that the cam look like garbage.
The Lucas is just what we use and seems to help.
Veno has an awsome thread on Zddp stickied at the top of drivetrain and performance.