Here is another thing to look at, read the plugs. If the engine has been running for a while, the offensive cylinder will have
oil deposits on the plug. If they all have deposits, check the draft system or something that may be dumping
oil in the intake system.
Could be something simple like a stuck PCV valve.
If you have an automatic with a vac. shift valve, check to make sure it has not gone bad. It will dump trans fuild into the engine.
Just a few more suggestions.