Car occasionally smoking blue

I have noticed that my car is occasionally (maybe once a week) smoking blue smoke from the exhaust. Not just a short puff. I don't see it often enough to know exactly what is going on, but I'll say for about 5 minutes. I'm not sure if it would stop by itself, or if it always would keep on until the engine is turned off.

I need to add one liter of oil between oil changes, with I do about every 5,000 kilometers.

I don't think it's the piston rings, because it is only occasionally. And a mechanic checked for oil steam while idling the car, and he said it looked good. He says the engine looks clean from the inside. (As far as he can see by removing the oil cap.)

The car is a Chrysler Cirrus Lxi with 6 cilinder motor. Year 2000

Do a compression test on all cylinders 1st.
If good it's probably just bad valve stem seals.