Puff the Magic Dragon Wagon?
Dannie, plain and simple, too much crankcase pressure caused by "BLOW BY". The origin,? possibly piston rings, a burned piston we could go on guessing all day. A compression test will aide you in finding the rascal. Considering it is Blow-by, a wet dry test may not be needed. If it is as bad as you describe, you may be able to find the cylinder by removing the spark plugs and inspecting them. One will be quite oily. You could crank the engine and hold your finger over the plug hole and feel for the weak one. Disable the ignition system first. Then go to your favorite technican and get the real pope, ah scope. Let's hope it is minor. Happy Trails Beep Beep from Houston!!!
Donald, you know, the one with his foot in his mouth