First off the Crankcase Ventilation (CCV) system components are listed as replace every 50,000 miles in the owner's manual of my 88 XJ and sure it is the same for your 89 XJ. I personally am thinking I will replace the front and rear tubes on the valve cover every 30,000 miles while I am doing a tune up.
Here is a couple of web articles on the problem:
http://www.off-road.com/jeep/cherokee/xjtech/engine/40ltr/blowby.htm
http://www.off-road.com/jeep/cherokee/98/04_apr/valvecover/cover.html
The air is filtered in the air box and then enters the engine in that big 1/2 inch tube. It goes through the engine and leaves by the small tube in the rear and is sucked into the intake manifold to be burned. IF the small tube becomes clogged then you get air in the oil filter since the large tube is the only way for pressure to get out of the crank case.
I did the valve cover cleaning and replace the CCV system but still had a small version of the problem. I decided to have the cat coverter replaced at 125,000 miles and it made a difference in how much power the XJ had. I also noticed my few drops of oil in the filter proble was gone! I concluded the cat was making back pressure and throwing off the operation of the vacuum system.
I would do what the articles says - clean the valve cover and replace the CCV tubes. Then install a paper filter and monitor for a couple of weeks. If oil reappears on that new paper filter then go to your favorite muffler shop and have them change out the cat converter. That what finally solved my problem. Also do NOT accept the muffler's shop "hand over the tail pipe" test to figure out if your cat is bad. If you have over 100,000 miles is prob at the end of it's life.
One more thing when changing the CCV tubes you may want to clean the vacuum fitting on the intake manifold, it may be full of crud. A good cleaning we make sure the vacuum port is in good operating condition. A bonus would be to hook up a vacuum gauge and find out how much vacuum you have. I run 16-17 in Hg but if you are running 12-13 in Hg you have a dirty intake.
You also may want to have the throttle body cleaned once this CCV problem is fixed. When the throttle body is off, inspect the intake. I had mine done at the dealer and the mechanic told me I had a very clean intake but if your intake is dirty Chrysler has a foaming cleaning to get rid of all the build up. Remember you have been running with a bad CCV system, all that engine "stuff" has been passing through your throttle body, so expect to have it cleaned once you get the CCV back in working order.