Actually you are right in saying it is either the valve stem oil seals or bad rings, but compression can still be fine with a bad ring or oil seal. If it is a ring, it will be the oil control ring, usually a 3 piece ring that gets coated with carbon and sticks in one place. Notice if it only smokes during startup or all the time, or when applying stress (gas). If it is on startup only, is most likely it is oil seals. The heat from the motor causes the ring to expand thus doing it's job even when there is a lot of carbon buildup. If it is all the time or just during stress, most likely with either of the two, it is the oil rings. if you replace the valve stem seals, use the rope as it is much, much easier and reliable then air. Just use nylon (bungee works awsome) rope approx 2-3 feet long and coil it into the cylinder while turning and it will coil like a slinkey. Then bring the cylinder you are working on upto TDC and the rope will hold the valves inplace when you let go the keepers. Rings can be replaced while block is in the car.

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