First exmaine your engine to figure out where the leak is coming from.
If there is oil:
* On top of the valve cover then it a bad oil filler or the Crankscase vent hoses. I'd get those parts at the dealer.
* Are the valve cover bolts loose? The ones by the firewall were real loon on my 88 XJ. You may want to pull of the valve cover, clean everything then reassemble. Up to the mid 90s RTV was applied to the valve cover at the factory then cured. Find out the bolt torque and torque from the center out. Tighten all 15 bolts/studs, wait 10 minutes and do it again to verify the torque. You will need a deep socket, 11mm on my 88 XJ, and a torque wrench. Do not try to use the calibrated arm. The manual says to touch up the RTV with new if it is damaged. I installed a fel-pro cork/rubber gasket on my valve cover.
* Is oil coming from the oil filter mount? Go to the dealer and buy the set of three O-rings. There an article about changing the oil filter adaptor to accept a larger filter on older XJ, the article is at
www.madxj.com and has good pictures.
Once you exhaust those then you are left with the rear main seal, oil pan gasket and the timing cover gaskets. The rear main is a pain to do, you have to pull the oil pan.
If you have some fresh cardboard lay it on your driveway and see where you XJ is leaking. Follow back up. A leaking valve cover gasket can look more like a rear main seal leak than an actual rear main seal leak.
Remember oil flows down hill once it leaves the interior of the engine.