Using the aforementioned oil in the spark plug holes method the only result you will get will be whether the rings or something higher is leaking. If the lower compression is not changed with the introduction of oil into the cylinder you know that the rings aren't leaking. Armed with that knowledge the actual leaking item, intake valve, exhaust valve, or head gasket, is irrelevant because the repair involves removing the cylinder head and performing a valve job which would also include replacing the head gasket. For me that test is a type of go-no go indicator. Am I pulling heads or the whole motor? I recommend a full valve job on both heads if one valve is causing the low compression, just to be on the safe side. You don't want to do one head and then 6 months later have to do the other head. As a matter of fact, the only repair mentioned earlier that would be just a remove and replace operation would be a defective head gasket, however even if the only issue is a leaking gasket I would still at least have the heads decked and depending on how many miles are on the motor I would also likely perform a valve job since the head was off the vehicle anyway. If you are that far into the engine I feel the extra maintenance is a pretty good way to ensure reliability in the future. Just me .02 on the whole leak down testing and subsequent repairs.