the newer oil without the zinc is a deathwish in a flat tappet cam.
your engine builder going to rebuild your 22k mile flat tappet camed engine when the cam goes flat? as it wipes the cam and valve tips and rockers.. all parts that require the zinc and when without it, open up the lash on the valvetrain preload.. as all these part wear?
next up is man made oil.. flat tappet cams REQUIRE some friction to spin the lifter.. the syn oil remove that.. a lifter that doesn't spin goes flat ..
g.m. engineers know a little more than that machinist..
if there wasn't a reason for spec'n an oil they'd not waste the time or the dollars doing so..