My friend bought a great looking van, cheap, but about a year after he got it, it started leaking tremendous amounts of oil out of the valve covers, and the PCV line was corked up with oil and crud. The oil had sludged up and corked up the drainback holes in the heads, an when it was driven over about 25 MPH for any length of time, the oil started pumping out. 
My friend teaches autoshop, and it was a great demo for what happens when you don't change the oil. The engine was sluged up tremendously, and he had his students yank it, and clean out as much crud as was possible and then run Mobil 1 synthetic in it, changing it after 3 days the first time, working up to 3 month changes eventually. The first change produced about a quart of sludge that the kids couldnt get out. 
I saw less loaded up engines clean up with Mobil 1 myself, and the first oil change was amazing, you wouldn't think all that crap could come out just by changing the oil.
Another friend bought a 2 year old Ford F150 from some guy and as far as we can tell, the oil was never changed, period.  Apparently, he thinks oil changes are an oil company "plot". There are kooks everywhere..