Right. Oil gets into the combustion chamber one of two ways..either by the piston rings, or via the guide seals. (Rarely also by an oil passage adjacent to the cylinder head/block interface...i.e. defective gasket). A compression check will give you a rough idea of the clearance piston skirt vs cylinder wall, altho if ok, it doesnt completely rule out broken rings or oil control rings stuck in grooves. Puffs of oil from idle in my humble experience (depends on mileage here) usually is the guide seals. These are rubber seals that harden up at 125-150K miles, and begin to leak oil into the combustion chamber, then when you load the engine up and bring the rpm up, it blows the oil out, hence the puffs of smoke. The rubber just hardens up over time. It clears up because the oil is not pooling in the area above the guide seal, after takeoff.

Unless this engine is unusual, you should be able to change the seals without pulling the head. You use an air fitting at the plug hole and keep the chamber pressurized, while you depress the valve spring, remove the keepers, and replace the seals. The only downside of this is if there is excessive clearance guide to valve, you won't be fixing this at this time. I have found that this works for engines with 100-150K on them or so. If the mileage suggests new guides or valves (poor sealing of valves, say burned valve edges, would show up as poor compression), then the head needs to come off, and this would be the ideal time to do the valves and guide seals.

I would do a compression check, and depending on the results, and mileage, consider either the guide seals, and/or valve job. I would pull the pistons out after ruling out the top end. If this car has mega-mileage, maybe so.

(BTW...you are sure some of this oil loss is not simply leaking out the bottom somewhere when the engine heats up?)

hope this helps dave