blue smoke at startup

[gman]

Just wondering what the likely culprit might be of my mild blue smoke at startup. It always does it after sitting overnight but not during normal stop and go daily driving. Is it bad oil seals causing oil to run down valve guide into combustion chamber? Doesn't smell during normal driving, just at startup. Tuned using vacuum guage, 18 inches of vacuum at 5000 feet elevation. Runs well and starts everytime (knock-on-wood).

Any other ideas or disagreements on the valve seals. 170K on original motor. Thanks.