Valve seal issues are typically limited to start-up and only a few seconds after that.
Mechaanical problems with rings, pistons, etc, usually don't go away with warm-up.
I'd suspect you have aleaky gasket that closes up tight once the engine warms up. Intake manifold gaskets are a natural "first look". Could also be a head gasket, but that's not as likely.
There's exceptions to all I wrote, but typically, I think I am correct.
1/ Althought intake gaskets can leak from the bottom (where the
oil is), you can try squirting a little WD40 around the manifold and see what haappens.
2/ A compression check won't do much, but a leak-down test can tell a lot when it comes to leaks.