Changed spark plugs and one old one covered with oil. What is the problem?

Not good. If you are saying the inside part of the spark plug is covered with oil, one of several things has happened. A stuck oil wiper ring on the piston will let oil bypass the rings. More likely you have a blown head gasket or cracked head. Has the engine overheated recently? When engines overhead, heads crack from lack of coolant. One was to veryfy this problem is to look at the oil on the dipstick. If the oil is frothy brown like a thin chocolate mikkshake or if you see small drops of oil clinging to the dipsitck along with the oil, you have either blown a head gasket or have a crack in the head or engine block. Also check the coolant in the radiator. If it has oil or an oily sheen in the water it also indicates the same problems. I assume you are having a miss when your engine is running and probably some blue smoke from the exhaust pipe. There is no simple fix for these problems. If you or a friend has a compression tester, remove the oily spark plug and a couple of other plugs. Turn the engine over several times with the compression gauge in the spark plug hole. It the pressure differs by as much as 10% or more you have the big problems as mentioned above. As much as you may love your 97 town car, it may be time to let it go. My estimate to do a head gasket would start around $900.00 or higher. Good luck.

If you notice smoke when you first start the engine, it may be that the rings are worn,or the valve guides in the heads are worn, and the valve seals may have failed. Also the oil may be cavitating in the valve covers, and not running down to the pan, as the engine is running.The oil drain holes may be plugged up from sludge! And the pcv valve may be plugged! Verry high mileage,not enough oil changes?