Oil in the vacuum advance canister and tube

[Texasdoc]

Here is one I can't figure out.

351W with MSD 8478 distributor. Swapped the vacuum advance canister for an adjustable one. Works fine. Vacuum line is connected to the manifold vacuum port on the bottom of the carburetor. While recently trying to figure out why the engine runs rough, I disconnected the vacuum line where it attaches to the canister. I noticed that there is oil in the vacuum line. I'm not sure where it is coming from.

Only thing I can think of is that it is coming up from the valves (or down thru the valve stems from the heads), and going back up to the carburetor. Although I don't see any oil when I take off the carb.

I do have a PCV valve connected to the main vacuum port on the back of the carb, but there is a catch can in the middle of that line, and it stays bone dry. Plus, this comes into the carb in the back by the secondaries. It would have to go down into the intake, then back up by the primaries, then thru the small hole that provides the source for the vacuum port.

The inside of the distributor is clean. I just don't get it.