Really good background information. Very simplified, but that's fine.
ZDDP is an additive that is consumed during engine use. It's consumed when it prevents wear in a metal on metal contact situation. When there is less to begin with, it will be consumed more quickly. So, in a flat tappet engine with high valve spring pressure, you'd want to change it more often.
The Valvoline guy in no way could say to use his VR1 racing oil in a street engine. It says right on the label off road use only, or something to that effect. To preserve catalytic converters, it must. But it's fine for a pre-cat car.
Bottom line is that for the OP with his 318, he'll probably be fine doing just what the Valvo guy said. But in my never apart 56K mile 340, I'm using the Quaker Hi Mile 10W-40 with 50% more ZDDP.
