I have had a PO300 for the last two months & now it has turned to a PO306. on 2002 Cadillac DeVille DTS

Do I have two errors or has the real problem finally showed up? I get the 300 @ start up and about one min. later the 306 shows up.

Asked by for the 2002 Cadillac DeVille DTS
Po300 is a misfire code not identifying the cylinder, Po306 identifies number 6 cylinder as being the cylinder that has the misfire. It could be spark plug or ignition related, fuel injector related or valve seeing issue/engine mechanical. Unable to tell without being able to do diagnostic tests.