Could be intake manifold air leak. Squirt some carb cleaner along the sides with engine at idle. If idle changes, you are sucking air and
oil in from the lifter gallery. Also check torque on intake manifold bolts, they tend to loosen. This happened to me on a 351W with FelPro black rubber intake gaskets and cork front/rear. I was using a quart of
oil every 500 miles with no smoke and no obvious leaks. I reinstalled intake with Cometic gaskets and RTV front/rear and
oil consumption returned to nil.