You are running dual quads, and the fuel mixture is too rich, there is a big surprise. Replace the sparkplugs, drive it hard, then pull them out and take a look. Also, you should put a capacitive discharge box on there , such as an MSD or Jacobs. It is hard to tell if your problem is
oil related, or just way too much fuel, since the problem is worst on the rear two cylinders, and you have dual 4bbl's, I am guessing fuel.