im runnin kinda same problem just fixed plenum repair but its still buring oil and number 8 cyclinder is low on compression as well

 

You get a CEL for it? I did before the fix and after for awhile then no CEL.
Cracked head, seat, or as Hey You said valve seal/s.

 

I did the plenum fix several years ago with a number 8 misfire before and after. It took about 4 seafoam treatments and an engine flush to break the rings free of all the carbon and oil sludge. Misfire is gone, has been gone. I do think soon I will need valve stem seals. I recently started getting the trademark blue smoke at start up of that particular issue.

 

You got way more nerve than I do. I bought the engine flush but got scared away from using it.