passed leak-down test, oil burning mystery

[peterb]

I have a 1975 GMC with 350, auto trans. Have been burning oil. Blue puff at start-up but donut otherwise see smoke. Replaced valve stem seals. No change. Checked, some oil in plugs 1&3. Did cylinder leak test with mechanic. All cylinders ok. Checked (intake) valve seals again, looked fine. Didnpt check exhaust valve seals (though these were replaced 6 mo. ago along with intakes. Cannot figure source of blue cloud. If oil were leaking from gasket below say manifold wouldn;t truck be smoking all the time, not just blue puff at startup....thought it was rings but since leak test checks out fine....I'm puzzled.

[Captkaos]

generally this is from leaking valve seals...