Poor compression in one cylander at idle.

I have to go with these guys, since the oil raised the compression.
Oil won't raise it if it's a valve usually.
Only if it's rings.

I also have to agree with the mechanic, (ring/piston or cyclinder wall problems)

krazykuda said

I'm going to go with rings....

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Oklacarcollecto said

I agree. The compression jump with oil in the cylinder and the leak down test would make me think the valves are sealing

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