Either the breather in the valve cover is clogged, or you've got some serious blowby. Pull the tube off the top of the valve cover. If you can feel air coming through it while the engine is running, then you know that the breather is clean.
Next, blow through the tube that you just pulled off (I'd say with your mouth, rather than compressed air, incase something is really clogging it, you wouldn't want it to go into the intake). If it's free, then you know that the breather system is venting crankcase pressure back into the intake. The only conclusion is that you've got some wicked blowby.
Either way, it really shouldn't be pushing oil out. Are you overfull?