If the oil made no difference, then you are likely correct, it's related to a valve not seated fully or damaged.
I assume this test was done due to a misfire. Were there any other symptoms you noticed such as white smoke from the exhaust, or any overheating issues? It could be a head gasket issue, but that is a big difference in cylinders.
Take a look through this link and let me know if anything mirrors something you remember:
If not, the next thing to do would be to remove the valve covers and see if there is anything pointing to a stuck or damaged valve.
Let me know.
Tuesday, May 10th, 2022 AT 8:14 PM