This is caused due to worn piston rings. You need overhaul the engine
a stopped up PVC system will cause this also.
Worn valve guides can also cause this. Have a wet and dry compression check done
to determine if you need a valve job or a rebuild.
The causes of the overpressure are several. The easiest (and cheapest) one to check and fix first is the PCV (positive crankcase ventilation) valve. If you put in a new one, my bet is that your problem will be solved.
However, if a new PCV valve doesn't get it done, you have more expensive issues at hand. The two primary causes of crankcase overpressure (other than the PCV valve not working) are, as previous posters said, broken/worn piston rings and worn valve guides and seals. Either condition will cause overpressure in the crankcase, and oily "smoke" to barf out your dipstick tube.
Every customer I had come in with clogged crankcase ventilation system problems, I started with the PCV, then went to a compression check. It's always important to run a vacuum check also. This will tell you if it's bad valves. If the compression is dead in one cylinder, take a thin steel rod, or maybe a straightened coat hanger and tap around the edges of the piston, then across the head. I've had a few that for one reason or another, had holes in the piston, causing a lot of blow-by.