Blue smoke but intermittent and rare 2000 Mazda Protege?

I have a 2000 Mazda Protege (DX model, 1.6L, manual trans) that on rare

occasions spews blue smoke out the exhaust. It does not do this on startup, so

valve seals don't seem to be suspect. It only happens when the car is warmed up

and has been running for a while, and not continuously. When it happens, it

looks like I'm driving an old oil-burner. Then it stops.

Since it's intermittent, and rare, I would think that piston rings would also

not be suspect. If they end up being the culprit, I'm just going to put up with

the problem since the car doesn't use enough oil to make it expensive to keep it

topped up in oil. I don't know how much oil it uses per 1,000 miles, but it may

use a quart every 1-2 K miles. Not sure on this, though.

I'm wondering if there's some emissions equipment that might not be venting the

crankcase pressure. I'm not seeing the MIL (check engine) light turn on to

indicate anything. I do have a code checker, so I can check for codes if there

may be some that don't turn on the light.

Any suggestions on where to start looking for the reason for the smoke?

My guess is that the smoke is oil-related and smells like it. It's not coolant.

But I don't know how the oil is getting into the pistons and getting burned

since this doesn't fit the usual scenarios that I'm familiar with.