I can think of a couple.
Most likely it's an intake valve seal. Which is usually a rubber O ring between the intake valve stem and the retainer. Sometimes they can break and fall out.
Some times a loose piece of valve cover gasket can plug a drain back hole and flood the valve area with oil. But this usually causes oil fouling at the rear of the engine as that is the low end of the engine.
The rings or ring lands can break, If it's a ring land it might not show up on a compression test.
The intake manifold can leak oil from the valley area into the port. This would be accompanied by a miss and spitting back while idling.
If a wrist pin came loose it might contact the cylinder wall and put a groove in it leaking oil, but compression would be down too.
Most likely it's the valve seal. If you have the type that goes over the valve guide, it could be worn out.
Either way, you can put air into the spark plug hole and change the seal and add a push on seal over the valve guide. The intake side is the one to worry about.
Check my web page for suspension info:
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer